Associates with spermatozoa in the epididymal fluid but does not bind tightly to them. Binds both all-trans and 13-cis retinoic acid. May act as a retinoid carrier protein which is required for epididymal function and/or sperm maturation. Synthesized by the mid and distal caput of the epididymis and secreted into the epididymal lumen. Epididymal fluid of the caudal and corpus regions (at protein level). Down-regulated to 11% of control levels 5 days after castration and is not detectable by 20 days (at protein level). Levels are partially restored by subsequent testosterone treatment. 2 different forms with differently processed N-termini exist. Belongs to the calycin superfamily. Lipocalin family.