Featured Article: Gao XH, Cui YY, Levenson RM, Chung LWK, Nie SM. In vivo cancer targeting and imaging with semiconductor quantum dots. Nat Biotechnol 2004;22:969–76.2 The development of high-sensitivity and high-specificity probes that transcend the intrinsic limitations of organic fluorophores has been highly sought after by scientists in fields ranging from basic biology to molecular imaging and medical diagnostics. Since the first demonstrations of their bioapplications in 1998 (1, 2), fluorescent semiconductor nanocrystals, commonly known as “quantum dots” (Qdots), have been a particularly attractive candidate for analytically sensitive, multiplexed, and quantitative optical imaging. Remarkably, after 15 years of research, the interest and efforts in engineering multifunctional Qdot probes are still growing, because of Qdots' combination of superior optical properties, the unique size and surface effect of nanomaterials in biological systems, the advantages offered by optical imaging, and the challenges of understanding and solving the nanotoxicity problem. Compared with organic dyes, Qdots exhibit size-tunable and spectrally narrow emission properties, efficient light absorption across a wide spectrum, outstanding photostability, and …