May be involved (but not essential) in spermatogenesis. Interacts with CCT5. Localized predominantly in the nucleus of haploid round spermatid. Testis specific (at protein level). Specifically expressed in spermatids; Sertoli cells maintain the level of expression in spermatids. If isolated spermatids are cultivated for 16 hours alone, the expression of THEG is down-regulated. May require signals from Sertoli cells to initiate changes in its gene expression through spermatogenesis. Expression in testis detected after stage P20, when haploid germ cells appeared in testis. According to PubMed:10747865, mice lacking Theg (mutant kisimo) have virtually no spermatozoa in the lumina of seminiferous and epididymal tubules. Spermatids in the vicinity of the lumina of seminiferous tubules showed vacuolation and were occasionally phagocytosed by Sertoli cells. Elongated spermatids have abnormal or completely nonexistent flagella. No difference is seen between wild type and null mutants in the number of spermatogonia or spermatocytes. According to PubMed:12748127 null mutants appear phenotypically normal and were fertile. However, a minor but significant reduction in testes weight was observed. Morphological appearance of sperm was normal. 'Kisimo' is a Japanese word for goddess of easy delivery.