SCL^−/Δ mice have increased bone marrow and splenic mast-cell progenitors. (A) Morphology of typical colony grown from agar culture of SCL^−/Δ bone marrow. The colony was floated and fixed on a slide and then stained with hematoxylin. (B) Toluidine blue stain of cells picked from a typical colony and spun onto a slide. (C) Flow cytometric analysis of a single colony stained with c-kit, Sca-1, and FcϵRI. The dotted line in the histogram represents the isotype control. (D) Acetylcholinesterase stain of floated and fixed mast cell colony demonstrating megakaryocytes (Megs). (E) Flow cytometric analysis of CD34 and β7-integrin expression on lineage marker–negative (Mac-1, B220, TER119, CD3), c-kit⁺ Sca-1⁻ IL-7R⁻ bone marrow (BM) and spleen cells from SCL^+/Δ and SCL^−/Δ mice. The numbers represent the mean percentages within the gates of total nucleated cells from 2 mice of each genotype. (F) Mean colony numbers from bone marrow cells cultured in methylcellulose for blast-forming unit–erythroid (Ery) or agar for megakaryocyte (Meg), granulocyte/macrophage (G/M), and mast-cell colonies. Numbers have been standardized for 25 000 cultured cells from mice reconstituted with either SCL^+/Δ or SCL^−/Δ bone marrow cells (n = 4 for each genotype).