The majority of Dnmt3l-null pachytene spermatocytes are transcribing X or Y genes (MSCI failure). (a and b) RNA/DNA FISH showing strong Zfy2 transcription in a spermatocyte with multiple γH2AX domains (note that some of the γH2AX staining has survived the DNA FISH procedure). (c and d) A rare pachytene spermatocyte with no Zfy2 RNA FISH signal in which the Zfy2 locus (arrow) lies within the sex body–like γH2AX domain (white outline). (e and f) An H1t-positive spermatocyte showing Zfy2 transcription. (g and h) A pachytene spermatocyte with multiple γH2AX domains that is strongly expressing Ddx3x/Usp9x. (i and j) A pachytene spermatocyte with no Ddx3x RNA FISH signal with the locus within the sex body–like γH2AX-positive domain. (k and l) An H1t-positive spermatocyte showing Ddx3x/Usp9x transcription. (a–l) White outlines indicate the extent of the γH2AX domain before the DNA FISH, and arrows point to the FISH signals (either RNA FISH or DNA FISH). Bar, 5 μm.