Numerous nucleic acid ligands, also termed decoys or aptamer, have been developed during the past 20 years that can bind to and inhibit the activity of many proteins. The concept that nucleic acid ligands could modulate the activity of proteins emerged from basic science studies of viruses and early work in the field of gene therapy. In the 1980s, research on HIV and adenovirus discovered that these viruses encode a number of small structured RNAs that bind to viral or cellular proteins with high affinity and specificity. For example the human immunodeficiency virus has evolved a short, structured RNA ligand the HIV TAR element that binds to the viral protein tat as well as the cellular protein cyclin T1 to control viral gene expression and replication. Similarly adenovirus has evolved a short structured RNA aptamer, termed VA-RNA, to inhibit interferon induced PKR activity and thus block one of the mammalian cell’s antiviral strategies (O’Malley et al., 1986; Burgert et al., 2002). The observation that viruses utilize RNA ligands for their ends suggested to those working in the field of gene therapy in the late 1980s that RNA ligands might also be useful for therapeutic ends. The first study performed to determine if an RNA aptamer could be utilized to inhibit the activity of a pathogenic protein was published in 1990. This groundbreaking work demonstrated that the TAR aptamer evolved by HIV to recruit viral and cellular proteins to viral transcripts could be turned against the virus to inhibit HIV replication (Sullenger et al., 1990). CD4+ T-Cells containing the TAR aptamer were shown to be highly resistant to viral replication and cytotoxicity (Sullenger et al., 1990). Thus these studies demonstrated for the first time that RNA ligands could be utilized as therapeutic agents to directly bind and inhibit the activity of clinically relevant proteins. This approach to HIV gene therapy is still being explored today by Rossi and colleagues (Li et al., 2005; Anderson et al., 2007). In the same year as the discovery that RNA ligands represent a new class of therapeutic agent, Tuerk and Gold (Tuerk and Gold, 1990) and Ellington and Szostak (Ellington and Szostak, 1990) demonstrated that large libraries of RNAs could be screened in vitro for RNA ligands that bind T4 DNA polymerase and a variety of organic dye respectively. This in vitro selection process, was termed SELEX (systematic evolution of ligands by exponential enrichment) by Tuerk and Gold (Tuerk and Gold, 1990) and the resulting RNA ligands were given the name aptamers by Ellington and Szostak (Ellington and Szostak, 1990). This approach has been widely used during the past two decades to generate RNA ligands to many proteins (Reviewed in Nimjee et al., 2005). The focus on this review is on aptamers relevant for the treatment of cancer, particularly aptamers targeting proteins that could be relevant to tumor immunotherapy. Tumor cells vary from healthy cells quantitatively and qualitatively in their protein repertoire. These cells differ qualitatively due to their inherent genetic instability, which is brought about by defects in cell cycle or factors involved in replication of genetic information. This lack of control leads to the expression of a variety of defective cell products and protein mutations in their amino acid sequence. One such example is the receptor tyrosine kinase (RTK) RET. A point mutation (C634Y) has been shown to produce a constitutively active form of the receptor and has been implicated in the development multiple endocrine neoplasia (MEN) type 2A and 2B syndromes and familial medullary thyroid carcinoma (Pestourie et al., 2006). Aptamers to these targets have been developed through SELEX targeting purified protein constructs as well as cells expressing the protein target (Cerchia et al., 2005; Pestourie et al., 2006). Though these aptamers were shown to bind and inhibit the function of RET C634Y, and revert morphological changes in the morphology of fibroblast (NIH3T3) transfected with the mutant receptor, the compound’s in vivo function remains to be determined. Aside from the differences in quality, the quantity of protein expressed can also differ vastly in cancerous cells: expression can be upregulated or de novo induced. Together with qualitatively different proteins, these overexpressed proteins are referred to as tumor markers. These proteins can be used to detect the presence of cancerous tissue, directly affect tumor progression through manipulation of tumor marker function, or deliver therapeutic agents to the tumor. Therefore, many aptamers have been developed that target tumor markers. These include aptamers targeting MUC-1 (CD227) (Ferreira et al., 2006), prostate specific membrane antigen (PSMA) (Lupold et al., 2002), human epidermal growth factor receptor-3 (HER3) (Chen et al., 2003), and tenascin–c (Hicke et al., 2001; Daniels et al., 2003), liver carcinoma cells (Shangguan et al., 2008b), and protein tyrosine kinase 7 (PTK7) (Shangguan et al., 2008a) (Table 1). Most of these aptamers have or will be used for the detection of cancerous cells (PSMA, MUC-1, tenascin–c, PTK7, liver carcinoma) (Chu et al., 2006b; Ferreira et al., 2006; Hicke et al., 2006; Borbas et al., 2007; Smith et al., 2007; Shangguan et al., 2008a; Shangguan et al., 2008b), or as more recently as targeting moiety for the tumor specific delivery of other therapeutic agents including radioisotopes (PSMA) (Farokhzad et al., 2006; Borbas et al., 2007), toxins (PSMA) (Chu et al., 2006a), or siRNA (PSMA) (Chu et al., 2006c; McNamara et al., 2006). Table 1 Targeting tumor markers with aptamers To evaluate the potential usefulness of other receptor-targeting aptamers (such as MUC-1) for the delivery of intracellularly acting therapeutics (such as siRNAs), each receptor will have to be individually validated, as there are vast differences in mechanism for receptor internalization. Internalization may be a function of receptor turnover or could also be activation dependent. This can potentially result in large differences in receptor-mediated uptake efficiency, that will affect the amount of intracellular therapeutic delivered. An additional layer of complexity is added, as the therapeutically necessary amount of intracellularly acting compound to be delivered may differ widely. Only one of the tumor marker specific aptamers above mentioned has been demonstrated to have antagonistic activity (HER3) (Chen et al., 2003). This aptamer inhibits the growth of breast carcinoma cell line MCF 7. Unfortunately, its therapeutic potential is limited by its susceptibility to nuclease degradation as it is composed of natural RNA. Aptamers for the modulation of immune responses Cancer immunotherapy is based on the observation that tumor cells vary from healthy cells quantitatively and qualitatively in their protein (and potential antigen) repertoire. Cells of the immune system can in principle recognize unique tumor markers as antigen and they are therefore often referred to as tumor-associated antigen. In an ideal scenario, the generated response would be sufficient to destroy the tumor and indeed this is sometimes the case in the very early stages of tumor onset. Unfortunately, the strength of this type of response is usually weak and worse yet, tumors develop mechanisms over time that lead to evasion of immune recognition. These mechanisms include the expression of inhibitory ligands to induce down-regulation of immune function through negative co-stimulation (Dong et al., 2002; Chen, 2004). The modulation and particularly the enhancement of tumor immune responses is of great therapeutic interest in the context of tumor immunotherapy. One approach has the potential to block undesired immune down-regulation to enhance therapeutic efficacy; another is to engage stimulatory signals. Antibodies, immunological receptors, cytokines, and chemokines have been targeted primarily through the development of antibodies or soluble version of receptor ligands or recombinant cytokines. Thus far, only few aptamers has been developed targeting immune modulatory proteins. Most of these were developed to inhibit immune responses and intended for application for the treatment of autoimmune diseases rather than the treatment of cancer by enhancing anti-tumor immune responses.