Transcription regulator of both male and female germline differentiation. Suppresses genes involved in spermatogonial stem cells maintenance, and induces genes important for spermatogonial differentiation (PubMed:22056784). Coordinates oocyte differentiation without affecting meiosis I (PubMed:28504655). Forms both hetero- and homodimers with SOHLH2. In males, it is mainly expressed in testis, while in females it is mainly expressed in ovary. In testis, it is exclusively expressed in spermatogonia, with a preference for prespermatogonia and type A spermatogonia. In ovary, it is detected in germ cell cysts, primordial follicles, and primary follicles but is undetectable by the secondary follicle stage (at protein level). Expressed in the majority of spermatogonia in adult animals, but not in the most undifferentiated spermatogonial population (PubMed:22056784). In male testis, it is expressed as early as 12.5 dpc. After birth, it localizes to type A spermatogonia in 7-day-old testis and adult testis, but not in spermatocytes. In spermatogonia, it is initially detected in stage IV Aal spermatogonia and strongly expressed in Aal, A1, A2, A3, A4, intermediate and type B spermatogonia (at protein level). In ovary, it is detected at 15.5 dpc, when oocytes have entered meiosis I, although a low level expression is detectable at 13.5 dpc. Expressed in oocytes of germ cell cysts as well as primordial follicles in the newborn ovary. In adult ovaries, it is preferentially expressed in primordial oocytes but disappear rapidly as the oocytes are recruited to form primary and secondary (multilayer and preantral) follicles. During oocyte differentiation, protein appearance at 15.5 dpc correlates with SOHLH2 translocation from the cytoplasm into the nucleus and is dependent on SOHLH2 expression (PubMed:28504655). Transcription is activated by DMRT1 in undifferentiated spermatogonia. Knockout mice are infertile. Males lacking SOHLH1 display disrupted spermatogonial differentiation into spermatocytes and show a strong down-regulation Lhx8 and Neurog3/Ngn3 genes (PubMed:16564520). Females display perturbed follicular formation, probably partially due to down-regulation of Nobox and Figla, 2 genes required for folliculogenesis (PubMed:16690745). Fertility could be restored by conditional expression of a SOHLH1 transgene after the onset of meiosis (PubMed:28504655).