Analysis of the biologic activity of mutant KitlSl-20Jprotein. (A) Flow cytometric analysis of fetal liver stromal cells derived from KitlSl-20J homozygous embryos (right) at 14.5 dpc demonstrates surface expression of Kitl protein. Fetal liver stromal cell line Sl/Sl4 derived from KitlSl/KitlSl embryos that lack expression of Kitl²  is used as the negative control (left). (B) Immunoblot analysis of conditioned medium from transfected CHOK1 cells. Conditioned medium was tested for expression of Kitl by immunoblotting with anti-murine SCF antibody. In lane 1, conditioned medium from nontransfected CHOK1 cells shows absence of Kitl. Lane 2 contains 15 μL conditioned medium from wild-type clone; lane 3, 30 μL conditioned medium from mutant (Kitl^Sl-20J) clone. (C) Proliferation of MO7E cells in response to conditioned medium containing WT and Kitl^Sl-20J proteins, measured by thymidine incorporation assay. Recombinant rat SCF and conditioned medium from nontransfected CHO cells (CHO) were used as controls (n = 6, mean ±SD; ^*P < .01, WT vs Kitl^Sl-20J).