Long-term culture (LTC) assay shows elevated colony production for HAT^ptmut cells. Lineage⁻, Sca1⁺ wt or mutant p300 BM cells were sorted and plated on top of irradiated stromal layers with the indicated genotypes. Bars indicate the average number of colonies produced per 1000 input cells ± SD; n = 3 to 6 independent wells for each bar, using BM from 3 to 6 mice, and 2 independently established stroma layers per genotype. *P < .05 by 1-way ANOVA of all possible pairwise comparisons (Tukey-Kramer test) was used to determine significance.