BLM and Bloom syndrome: KSVS1452 shows no detectable BLM protein and exhibits cellular defects characteristic of the Bloom-syndrome-patient-derived cell line GM08505, including increased levels of sister-chromatid exchanges, hypersensitivity to DNA-damaging agents, delayed repair of replication-dependent DNA double-strand breaks, and increased doubling time (Supplemental Fig. S1A–F), confirming successful inactivation of BLM in KSVS1452.