(A) Schematic representation of the development of male germ cells in the mouse from the primordial germ cells of the embryo, prospermatogonia of the fetus, postnatal mitotic spermatogonia, meiotic leptotene (L), zygotene (Z) and pachytene (P) spermatocytes, round and elongating spermatids to adult spermatozoa. The age of the mouse is marked in days post coitum (dpc) before birth and in postnatal days (P3, P6, etc.) after birth. (B) Z-DNA removal occurs at the initiation of global de novo methylation in the male germ line. Global level of DNA methylation across developmental stages is depicted by the dark blue line. DNA methylation reaches the lowest level at 13.5–15.5 days post coitum (dpc) and a burst of de novo methylation takes place between 15.5–17.5 dpc, and methylation levels remain high until the spermatozoa stage. ZBTB43 expression and Z-DNA removal is detected at 15.5 dpc. ZBTB43 is required for removing Z-DNA structure at PPRs, as shown by immunohistochemistry of wild type and Zbtb43^−/− prospermatogonia.