Human mast cell development in hSCF Tg NSG recipient stomach, small intestine, and large intestine. H&E- and anti–mast cell tryptase antibody–stained sections of (A) non-Tg NSG recipient stomach (NSG control, N1-3), small intestine (N5-1), and large intestine (N9-1) and (B) hSCF Tg NSG recipient stomach (S1-3), small intestine (S12-3), and large intestine (S12-3) demonstrating the presence of human mast cells (N1-3, killed at 24 weeks; N5-1, killed at 35 weeks; N9-1, killed at 20 weeks; S1-3, killed at 21 weeks; and S12-3, killed at 13 weeks). (C) Confocal immunofluorescence images of hSCF Tg stomach (S1-9) demonstrate human CD45⁺ (green) and human CD117⁺ (red) mast cells. (D) Frequencies of tryptase⁺ cells were quantified by sampling 3 areas each from hSCF Tg (n = 3) and non-Tg (n = 3) NSG recipients: hSCF Tg NSG recipients, 7.0% ± 0.6%; and non-Tg NSG recipients, 2.5% ± 0.5% (P < .0001 by 2-tailed t test).