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CaBP1–8 are neuronal Ca²⁺-binding proteins with similarity to calmodulin (CaM). Here we show that CaBP4 is specifically expressed in photoreceptors, where it is localized to synaptic terminals. The outer plexiform layer, which contains the photoreceptor synapses with secondary neurons, was thinner in the Cabp4^−/− mice than in control mice. Cabp4^−/− retinas also had ectopic synapses originating from rod bipolar and horizontal cells tha HJt extended into the outer nuclear layer. Responses of Cabp4^−/− rod bipolars were reduced in sensitivity about 100-fold. Electroretinograms (ERGs) indicated a reduction in cone and rod synaptic function. The phenotype of Cabp4^−/− mice shares similarities with that of incomplete congenital stationary night blindness (CSNB2) patients. CaBP4 directly associated with the C-terminal domain of the Ca_v1.4 α₁-subunit and shifted the activation of Ca_v1.4 to hyperpolarized voltages …