Human mast cell development in hSCF Tg NSG recipient BM. (A) Representative flow cytometric scatter plot and histogram demonstrating the identification of human CD45⁺CD33⁺CD117⁺ mast cells. (B) FACS-sorted hCD45⁺CD33⁺CD117⁺CD203c⁺ human mast cells from a representative non-Tg NSG recipient BM (N1-1, 0.9% human mast cells within hCD45⁺CD33⁺ population) and hSCF Tg NSG recipient BM (S1-3, 14.6%; S12-3, 8.8%; and S3-2, 7.3% human mast cells within the hCD45⁺CD33⁺ population) were examined by MGG staining (N1-1, killed at 21 weeks; S1-3, killed at 21 weeks; S12-3, killed at 13 weeks; and S3-2, killed at 15 weeks). (C) H&E- and anti-mast cell tryptase antibody–stained bone sections demonstrate hypercellular BM with high frequency of tryptase⁺ human mast cells in hSCF Tg NSG recipients. Non-Tg NSG recipient: N11-1, 70.7% hCD45⁺. hSCF Tg NSG recipients: S8-2, 99.6%; and S12-3, 79.5% hCD45⁺ (N11-1, killed at 20 weeks; S8-2, killed at 11 weeks; and S12-3, killed at 13 weeks).