A previously identified complex mutation, affecting exon 28 of the neurofibromatosis type 1 gene, was employed for the analysis of the expression pattern in primary cultures of neurofibroma cells and melanocytes from a café-au-lait macule of the patient, respectively. Reverse transcription and subsequent polymerase chain reaction amplification of the segment carrying the mutation revealed that both alleles were expressed in both cell types analysed, thus excluding loss of heterozygosity in this particular instance. Segregation of the alleles of the intragenic Alu sequence length-polymorphism disclosed the paternal origin of the mutated allele. Detection of this mutation was also used for presymptomatic direct DNA diagnosis in the younger child of the patient.