(a) Sanger sequencing identified a heterozygous Atm G2023R variant and a homozygous S707P variant in colonies of mK4 cells. (b) Western blot indicated that heterozygous Atm G2023R allele and homozygous Atm S707P change reduce of protein expression (relative ATM protein quantification by ImageJ). (c)Atm mRNA and protein are reduced in E12.5 mouse SCP treated with shAtm versus non-targeting (NT) control. (d) Photomicrographs of SCP spheres formed by wild type or Nf1−/− SCP with non-targeting control or shAtm, 4 days after plating at passage 5. (e) shAtm increases self-renewal of SCP spheres (2-way ANOVA, *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001). (f) shAtm increases SCP tumor number. A Fisher Exact test was performed. (g) Left, gross image of a xenograft tumor under the skin of a mouse grafted with Nf1−/−; shATM SCPs. The ruler shows 1 mm markings. Right, histological analysis of paraffin sections. The inset shows a higher magnification image of a de-granulated metachromatic (purple) mast cell in a toluidine (Tol.) blue stained section.