Intrinsic defect in Pot1b^Δ/Δ mice hematopoietic cells. (A) Scheme of competitive BM transplantation experiment. Two-month-old male and female wild-type or Pot1b^Δ/Δ mice were used as marrow donors. Female SCID recipient mice were lethally irradiated with 5 Gy of radiation. A total of 10 million mixed nucleated cells (50% from each genotype) were injected intravenously into the lateral tail veins of recipient mice. Recipients were killed after 3 months, and BM cells were analyzed with FISH for the presence of Y chromosomes. (B) Representative images of Y-probe FISH results in 2 different study groups. Group I: male Pot1b^Δ/Δ male donors, wild-type female donors. Group II: wild-type male donors, Pot1b^Δ/Δ female donors. Green: FITC labeled Y-FISH probe; blue: DAPI nuclear staining.