The only two known examples of function for BG molecules, regulation of actin-myosin by “zipper protein” in intestinal epithelial cells and effect of the BG1 gene on viral disease (32, 33, 36), are both associated with the cytoplasmic tail rather than the extracellular Ig-V domain, which may fit with the notion that the cytoplasmic tail is under selection for variation. The gene discussed is ACSBG1; the disease is viral load.