Plays a role in spermatogenesis (PubMed:14585816). When overexpressed, prevents nuclear localization of HIF1A (PubMed:16777103). Interacts with HIF1A. In mature spermatozoa, localizes to the centriole and midpiece (By similarity). The 27-kDa peptide associates with the fibrous sheath in mature spermatozoa and localizes to the principal piece of sperm tail, while the 55-kDa peptide localizes to the midpiece (PubMed:16643851, PubMed:16777103). Detected in the cytoplasm of almost all spermatogonial cells within the seminiferous tubules (By similarity). Predominantly expressed in testis, in spermatozoa (at protein level) (PubMed:14585816, PubMed:16643851). Not detected in Leydig cells (PubMed:16643851). The N-terminal 27-kDa fragment is also detected in liver, while the C-terminal 55-kDa fragment is also found retina, brain and kidney (at protein level) (PubMed:16643851). First detected at 7.5 dpc (PubMed:16643851). Tends to be expressed weakly during the early stages of embryogenesis. Expression increases over time in different growing and differentiating tissues, including brain (11.5 dpc), vertebrae (14.5 dpc), primordia of vibrissae follicle (15.5 dpc), intestinal sub-endothelium (16.5 dpc), pancreas (16.5 dpc), eyeball (16.5 dpc), liver (16.5 dpc), telencephalon (18.5 dpc) (PubMed:16643851). In the developing neural tube at 8.5-9.0 dpc, preferentially expressed in cells with one or more neuronal extensions (PubMed:16643851). During spermatogenesis, weakly detected in primary spermatocytes, but expression increases, reaching strong levels in elongated spermatids (stage V) and mature sperm (at protein level) (PubMed:16643851). Processed into N-terminal 27-kDa and C-terminal 55-kDa fragments. Belongs to the CEP135/TSGA10 family.