The h21 chromosome of Down syndrome mice, when unsynapsed, has unrepaired DSBs and is transcriptionally silenced but does not invoke pachytene losses. (a) Pachytene cell showing unsynapsed h21 (arrow), which is positive for BRCA1. (b) Pachytene cell with self-synapsed h21, which is negative for BRCA1. (c and d) The unsynapsed h21 is also positive for ATR and γH2AX (c; inset), whereas the self-synapsed h21 (d) is negative. (e and f) Human Cot1 RNA FISH/chromosome painting (arrows) showing that when h21 lies outside the γH2AX domain, it is transcriptionally active. (g and h) When h21 (arrow in h) lies within the γH2AX domain, it is transcriptionally silenced. (i) Pachytene cell showing RAD51 signals on the unsynapsed h21 and X chromosome; in the inset, the SCYP3 signal has been reduced so that the RAD51 foci are more visible. (j) In late pachytene, RAD51 signals disappear from both chromosomes. (i and j) In the insets, the SCYP3 signal is reduced to emphasize the lack of Rad51 staining. (k and l) In h21 carriers, apoptosis is not increased in stage IV tubules relative to controls (wt; arrows point to TUNEL-positive cells). (a–l) Arrows point to the h21. Bars: (a–d, i, and j) 10 μm; (e–h) 5 μm; (k and l) 40 μm.