Associates with spermatozoa in the epididymal fluid but does not bind tightly to them. Binds both all-trans and 9-cis retinoic acid. May act as a retinoid carrier protein which is required for epididymal function and/or sperm maturation. Synthesized in the proximal part of the epididymis and secreted into the epididymal fluid. Synthesized exclusively in the proximal part (caput epididymidis) of the epididymis. It makes up a substantial part of the total protein in the epididymal luminal fluid and binds to the sperm membrane. There are two similar, immunologically cross-reacting forms of this protein, designated B and C, which probably result from different processing of the amino end. The N-terminus of form C is probably blocked. Belongs to the calycin superfamily. Lipocalin family.