RBM4, also known as Lark, was described initially as having a role in circadian rhythm control in Drosophila. In the last 5 years data have emerged from studies of mammalian cells. It is now clear that RBM4 is an RNA-binding protein involved in diverse cellular processes that include alternative splicing of pre-mRNA, translation, and RNA silencing. Its structure, similar to other RNA-binding proteins, contains two RNA recognition motifs and a CCHC-type zinc finger. Here we review current information about the function of RBM4 and its localization within the cell. We then speculate about its possible relationship to disease.