PURPOSE: To analyze the spectrum and frequency of NF1 mutations in exon 10b. METHODS: Mutation and sequence analysis was performed at the DNA and cDNA level. RESULTS: We identified nine exon 10b mutations in 232 unrelated patients. Some mutations were recurrent (Y489C and L508P), others were unique (1465-1466insC and IVS10b+2delTAAG). Surprisingly, at the RNA level, Y489C causes skipping of the last 62 nucleotides of exon 10b. Another recurrent mutation, L508P, is undetectable by the Protein Truncation Test. CONCLUSION: As exon 10b shows the highest mutation rate yet found in any of the 60 NF1 exons, it should be implemented with priority in mutation analysis.