Musashi1 (MSI1) is a highly conserved RNA-binding protein that performs important functions in stem cell maintenance, nervous system development, and tumorigenesis. MSI1 inhibits the translation initiation of its target mRNA, m-Numb, by competing with eIF4G for the poly (A)-binding protein. MSI1 is mainly expressed in stem cells and progenitor cells from different tissues in mammals, and the MSI1 expression decreases as the cells commit to lineage differentiation. Recently, researchers identified MSI1 expression in mature photoreceptors and retinal pigment epithelium cells, and the MSI1 expression in these cells is absolutely necessary for the survival of photoreceptors. Abreu and his coworkers performed an RIP-Chip analysis and revealed that the MSI1-associated mRNAs belong to 2 main functional categories pertinent to tumorigenesis.