Mastocytosis in kit^D814Vflox Mx1-Cre animals. Giemsa-stained skin sections (mast cells dark purple, 5×/0.15 NA objective) revealed increased numbers of mast cells (A) in all mice and the presence of multiple mast cell skin tumors (B, 5×/0.15 NA objective) in 24 of 39 animals. (C) The mast cell tumors destroyed the dermal collagen as demonstrated by trichrome-Masson staining (green represents collagenous fibers; and pink, mast cells, 20×/0.7 NA objective). (D) Mast cells infiltrating a skin draining lymph node (Giemsa staining: dark purple represents mast cells; and turquoise, nonmast cell hematopoietic cells, 5×/0.15 NA objective) and (E) mast cell infiltrate in the submucosa of the forestomach (Giemsa, 20×/0.7 NA objective). (F) Flow cytometric analysis of a dermal mast cell tumor. A single-cell suspension prepared from a skin mast cell tumor was stained for kit and the high-affinity IgE receptor-α chain (FcϵRIα) and analyzed by flow cytometry.