The Nir/rdgB family of proteins has been identified in a variety of eukaryotic organisms, ranging from worms to mammals. The Drosophila retinal degeneration B (rdgB), a protein that is required for photoreceptor cell viability and light response, was the first to be identified. The rdgB mutant flies exhibit light-induced retinal degeneration, abnormal termination of the light response, and profound loss of electroretinogram (ERG) amplitude shortly after light exposure. The abnormality of their light response appears within hours after eclosion, while retinal degeneration is usually observed 3 to 4 days later. These observations implicated rdgB as an essential protein for photoreceptor-cell survival and light response. The finding that the N-terminal region of rdgB consists of a phosphatidylinositol (PI) transfer domain, which exhibits PI-transfer activity in vitro, suggests that rdgB is involved in phospholipid transport, metabolism, and signaling and thus play an essential role in photoreceptor membrane renewal and biogenesis. The other Nir/rdgB family members are functionally and structurally related to the Drosophila homolog, and are implicated in regulation of lipid trafficking, metabolism, and signaling. Prof. Sima Lev isolated Nir1, Nir2 and Nir3, the human homologs of RdgB.