(A). Analysis of bone marrow and EBV cell-line DNA from patient 6 and DNA extracted from the blood of his parents. Lanes 1 through 4 were analyzed using the polymorphic nucleotide repeat described by Andersen et al.36 Lane 1 shows maternal DNA, lane 2 shows paternal DNA, lane 3 shows EBV-line DNA, and lane 4 shows DNA extracted from leukemic bone marrow. The patient's EBV line, which is not involved in the leukemic clone, retains both parentalNF1 alleles, whereas the affected bone marrow sample shows a single allele consistent with LOH at NF1 in this leukemia. (B). Results of the IVTT assay in patient 6. Lanes 1 to 4 show polypeptides synthesized from RT-PCR product corresponding to exons 28 through 38 of the NF1 coding sequence. The sample in lane 1 is derived from patient 6; samples in lanes 2 through 4 show a normal protein pattern. The full-length polypeptide is indicated by an arrow. The truncated protein in lane 1, marked by an asterisk, represents the 6579 + 18 G to A mutation seen in the splice consensus sequence flanking exon 34 ofNF1 in this patient. The full-length IVTT polypeptide in this patient is represented by a fainter band than is seen in lanes 2 to 4, consistent with loss of the normal NF1 allele in this leukemia.