Genetic interaction between Kit and SCL. (A) Scl mRNA levels in purified MEPs from WT and W^41/41 mice. (B) Strategy used to test the genetic interaction between Scl and Kit. (C) The SCL transgene rescues the hematocrit of W^41/41 mice. Blood from 8- to 12-week-old mice was analyzed, and the hematocrits of individual mice and the mean of each group are illustrated. W^41/41 mice exhibit decreased hematocrits compared with age-matched controls (P < .05), which were corrected by the SCL transgene (P = .05). (D) Blood smears from 8- to 12-week-old mice. L, lymphocyte; Plt, platelets R, polychromatophilic erythrocyte (reticulocyte). Two hundred to 300 cells on 2 different slides were scored per mouse (n = 6 for W^41/41, and n = 3 for the other genotypes). (E) Analysis of progenitor-enriched populations by flow cytometry. The absolute numbers of progenitors per mice are shown as box plots for n mice of each genotype. *P < .05 compared with wt controls. (F) Bone marrow cells from wt, W^41/41, W^41/41-SCL^tg, and SCL^tg mice were cultured in methyl-cellulose, and the frequency of early (BFU-E) and late erythroid progenitors (CFU-E) as well as multipotent (CFU-GEMM) and myeloid progenitors (CFU-GM) was determined. Cultures were performed in triplicate, and data are shown as box plots with the medians as well as the 2 extreme values of each distribution. For CFU-GEMM, CFU-E, and BFU-E, the differences between wt and W^41/41, and between W^41/41 and W^41/41-SCL^tg were significant. *P < .05. In contrast, the number of colonies obtained from W^41/41-SCL^tg and WT mice was not significantly different (P > .1). (G) Gene expression analysis for MEPs from wt, W^41/41, W^41/41-SCL^tg, and SCL^tg mice. Expression levels of genes identified in Figure 4C were determined by quantitative RT-PCR. mRNA levels were normalized using Hprt as an internal control, and the expression levels in wt MEPs were set as 1. Shown are the averages ± SD of 2 experiments performed in triplicate (# not determined).