CABP5 is a calcium-binding protein primarily expressed in retinal bipolar cells that modulates calcium signaling in visual circuitry. 1 Its expression is restricted to rod and cone bipolar cells, where it appears during early post-mitotic development, preceding expression of other bipolar cell-type-specific markers. 2 At the molecular level, CABP5 belongs to a subfamily of neuronal calcium sensors structurally similar to calmodulin but with altered EF-hand motifs. 1 Single-cell transcriptomic analysis reveals that CABP5 is coexpressed with CaBP1 and CaBP2 in most bipolar cell types, with notably distinct expression in photoreceptors depending on species: mouse cones and human rods coexpress CaBP4 and CaBP5, while mouse rods and human cones express CaBP4 exclusively. 3 Transcriptional regulation of CABP5 involves paired-type homeodomain factors (Otx2, Crx) and POU-domain factor Brn2, which bind conserved promoter sequences to drive bipolar cell-specific expression. 4 While CABP5's precise functional mechanisms in bipolar cells remain incompletely characterized compared to other CaBP family members, its specialized retinal localization and developmental timing suggest critical roles in phototransduction and synaptic transmission within retinal circuits. 5