TI - Both ATM and XY Synapsis Are Dispensable for Meiotic Sex Chromosome Inactivation . AB - Sex chromosomes in spermatocytes are transcriptionally silenced during prophase through meiotic sex chromosome inactivation ( MSCI ) , during which the X and Y are included in the sex body , a heterochromatin domain that excludes the active ( PHOSphorylated ) form of RNA polymerase II ( reviewed in [37] , [38] ) ( Figure 4A-4C ) . PHOSphorylated RNA polymerase II was excluded from the sex chromatin of both Spo11+/- and Spo11+/-Atm-/- spermatocytes (Figure 4D-4F) , indicating that ATM is dispensable for MSCI . Importantly , MSCI occurred even when the X and Y did not synapse within the PAR ( Figure 4 ) , consistent with recent studies suggesting that MSCI is driven atleast in part by asynapsis per se [39] . The sex body is enriched for numerous proteins and protein posttranslational modifications [40] . One of these modifications is PHOSphorylation of the histone variant H2AX ( gammaH2AX ) , which has also been implicated in MSCI [41] . ATM is dispensable for gammaH2AX formation in the sex body [32] . Using FISH , we confirmed that the sex chromosomes were included within a gammaH2AX-positive even when they were not synapsed ( Figure S2A and S2B ) . When the sex chromosomes were separated , the X and Y were contained within separate gammaH2AX ( Figure S2C and S2D ) . Similarly , neither ATM nor PAR synapsis were required for localization of two additional sex body components , NBS1 and TOPBP1 ( Figure S2E and S2F ) . Thus , ATM is dispensable for formation of apparently bona fide sex bodies .