Testicular germ cell tumours (TGCT) of young adults arise from the intratubular precursor, carcinoma in situ (CIS). CIS cells are thought to be developmentally arrested and transformed fetal germ cells that survive through childhood and gain invasive capacity after puberty. Given that germ cell neoplasms arise frequently in undervirilized and dysgenetic gonads and the striking physiological difference between meiotic entry in ovaries (fetal life) versus testes (at puberty), this study aimed to investigate whether errors in regulation of meiosis may be implicated in the pathogenesis of CIS or its invasive progression to TGCT. The main focus was on a key sex differentiation and meiosis regulator, DMRT1, which has also been linked to TGCT risk in recent genetic association studies. Expression patterns of DMRT1 and other meiosis regulators (SCP3, DMC1, STRA8, CYP26B1, NANOS2, NANOS3) were investigated in pre- and post-pubertal CIS samples and TGCT by quantitative RT-PCR and immunohistochemistry. The results demonstrated that meiosis markers and meiosis inhibitors were simultaneously expressed in CIS cells, in both pre- and post-pubertal testis samples. DMRT1 was present in a restricted subset of CIS cells, which was relatively greater in pre-pubertal (27%) compared to adult (2.6%) samples. In contrast to the majority of CIS cells, DMRT1-positive CIS cells in adult testes were not proliferating. DMRT1 and most of the other meiosis regulators were absent or expressed at low levels in invasive TGCT, except in spermatocytic seminoma (not derived from CIS). In conclusion, this study indicates that meiosis signalling is dysregulated in CIS cells and that a key regulator of the mitosis-meiosis switch, DMRT1, is expressed in 'early-stage' CIS cells but is down-regulated with further invasive transformation. Whether this mixed meiosis signalling in CIS cells is caused by insufficient virilization of the fetal somatic niche or a partial post-pubertal maturation remains uncertain and requires further study.