Mutational analysis of the NF1 gene in JMML cells. All 60 coding exons including adjacent splice sites were PCR-amplified from genomic DNA of JMML granulocytes and sequenced bidirectionally. Mutations were confirmed by sequencing an independent, reamplified PCR product. (a) The analysis of sample D003 revealed a 2-bp deletion in exon 22 at nucleotide positions 3861 and 3862. This frameshift mutation results in synthesis of truncated neurofibromin. Homozygosity of the mutation is indicated by the absence of double peaks downstream of the frameshift. (b) Heterozygous single-nucleotide deletion at position 3366 in sample D127.