We have determined the status of DNA methylation at specific sites in three spermatogenesis–specific genes, Pgk–2, ApoA1 and Oct–3/4, throughout the development and differentiation of male germ cells in the mouse. We observed a specific demethylation event in the Pgk–2 gene in prospermatogonia at about the time of birth, about 10 days before the onset of transcription which first occurs in primary spermatocytes. All three genes were unmethylated in adult spermatogenic cells in the testis, but were remethylated in mature spermatozoa in the vas deferens. Surprisingly, we found that this remethylation is part of the process of sperm maturation which occurs in the epididymis.
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