Reduced Nf1 expression resulting from Nf1 heterozygosity. The amount of Nf1 transcripts was quantified by real-time PCR on cDNA obtained from total BM cells and spleen cell suspensions. Depicted is the mean relative expression ratio ± SD of Nf1 wild-type transcripts compared with expression of Gapdh mRNA (n = 5). Statistical significance (Mann-Whitney U test) is shown above the bars. (A) Icsbp^+/+. (B) Icsbp^−/−.