Neurofibromatosis type 1 (NF1) is an autosomal dominantly inherited disorder caused by mutations in the NF1 gene on 17q11.2. Melanocytes cultured from cafe au lait macules (CALM) of patients with NF1 were analysed for loss of heterozygosity (LOH) at the NF1 locus using a 3'-flanking and four intragenic markers. None of the informative samples showed LOH. In addition, the X-inactivation pattern of melanocytes from CALM (n = 4) and from the unaffected skin of the patients (n = 3) suggests a monoclonal origin of the cells isolated from skin biopsies up to 2 cm2 in size.