Fig. 3. (A) Spo11^–/– zygotene cells are devoid of S-foci. In contrast to wild-type (a) and Spo11^+/– (b) zygotene cells, surface-spread preparations of Spo11^–/– zygotene cells stained for SCP3 (red) revealed the presence of L-foci and a lack of S-foci on the axial elements (c,d). Spo11^–/– zygotene cells exhibited -H2AX signals that looked like L-foci (c,d). We defined two categories of zygotene Spo11^–/– cells: (c) without or (d) with a pseudo sex body (^*). In c' and d', zygotene stages of Spo11^–/– nuclei were defined by co-labeling of SCP1 (red) and -H2AX (green): SCP1-positive nuclei with a pseudo sex body corresponded to the most advanced stage (d'). Arrows indicate L-foci. Bars, 10 µm. (B) Change in the distribution of -H2AX L-foci during zygotene stage in Spo11^–/– mice. Zygotene nuclei from early (light gray bars) and late (dark gray bars) stages were classified according to their number of L-foci. A quantitative analysis of L-foci in Spo11^–/– zytotenes revealed that the most advanced cells (with a pseudo sex body) contained less L-foci.