We have previously described a patient with chronic myelomonocytic leukemia who exhibited a mutation (del-10:-8) in the splice-acceptor region in front of the FLR exon of the NF1 tumor suppressor gene. In order to evaluate whether this mutation indeed affects correct splicing of this exon we used an exon trap approach. Our data unequivocally prove the functional relevance of this NF1 mutation. Exon trapping thus represents an attractive strategy to study the consequences of putative splice-site mutations if RNA samples are not available.