SYCP1 and cyst: Mutant spermatogonia were positive for the undifferentiated spermatogonia marker, Plzf, which is expressed in single- to 8-cell (1- to 8-cell) cyst spermatogonia in the wild-type (Figure 1B; Ozaki et al., 2011), but negative for meiotic prophase I markers, Sycp1 and Sycp3 (Saito et al., 2014; Saito et al., 2011).