In accordance with these data and with SS being associated with defective ATR activation [2], [5], we also found that CtIPs/s and CtIP+/s cells exhibited a mild defect in ATR signaling as measured by Chk1 phosphorylation, with this again being more evident at low etoposide doses (Figure 2A; see Figure 2E for quantification). Here, CHEK1 is linked to synovial sarcoma.