Subunit of cyclic nucleotide-gated (CNG) channels, nonselective cation channels, which play important roles in both visual and olfactory signal transduction. When associated with CNGA1, it is involved in the regulation of ion flow into the rod photoreceptor outer segment (ROS), in response to light-induced alteration of the levels of intracellular cGMP (By similarity). Tetramer formed of three CNGA1 and one CNGB1 modulatory subunits. Isoform CNG4D is the most frequent isoform (CNG4D:CNG4C:CNG4E = 20:2:1) in testis. Retina, testis, kidney, heart and brain. In the rod cells, the Cngb1 locus encodes the cyclic nucleotide-gated cation channel beta-1 subunit and several glutamic-acid-rich proteins (GARPs). In the rod cells, the Cngb1 locus encodes the cyclic nucleotide-gated cation channel beta-1 subunit and several glutamic-acid-rich proteins (GARPs). Belongs to the cyclic nucleotide-gated cation channel (TC 1.A.1.5) family. CNGB1 subfamily.