During meiosis, asynapsed regions at pachynema are recognized by a mechanisms that recruits ATR, leading to the phosphorylation of the histone variant H2AX, which is thought to be indispensable to silence this regions. Thus, it is believed, that ATR function is necessary to silence those asynapsed regions of the genome (Meiotic Silencing of the Unsynpased Chromosomes, MSUC). However, this hypothesis has never been tested because ATR null mutation are embryonic lethal. The Lab of Dr. O. Fernández-Capetillo (CNIO) has developed an hypomorphic mutation of ATR which is compatible with life and will enable to test this hypothesis. Morerover, from the study of these mice, we will gather more information about ATR's function in mammalian meiosis.