UBL4B is a ubiquitin-like protein that functions in post-meiotic germ cells and mitochondrial regulation. This autosomal retrogene arose from X-linked UBL4A through retroposition during mammalian evolution and is restricted to post-meiotic spermatids and spermatozoa 1. UBL4B localizes to mitochondria in ejaculated sperm and serves as a downstream target of MAEL protein 2. The protein plays a critical role in mitochondrial function and ATP production; knockdown studies demonstrate that loss of UBL4B impairs mitochondrial integrity 2. UBL4B expression is significantly reduced in asthenozoospermic (low motility) sperm compared to normal controls, and its levels positively correlate with sperm motility parameters, suggesting involvement in spermatozoan function 2. Beyond reproductive tissues, UBL4B is aberrantly expressed in ovarian cancers despite its testis-specific origin, potentially offering diagnostic or therapeutic value 3. Additionally, UBL4B was identified as a potential biomarker associated with Parkinson's disease progression rates in peripheral blood 4, and has been implicated in colonic diverticulosis genetics 5. These findings indicate UBL4B regulates mitochondrial protein localization and function with broader pathophysiological implications beyond reproduction.