Recently, missense mutations in the PTPN11 gene on chromosome 12q24.1 have been identified as the cause of Noonan’s syndrome in 45% of familial and sporadic cases,4,5 indicating genetic heterogeneity within the syndrome. In the study by Tartaglia et al,5 there was a family in which three members had features of Noonan’s syndrome; two of these had incidental mandibular giant cell lesions.3 All three members were found to have a PTPN11 mutation known to cosegregate with the Noonan phenotype. This mutation, an A→G transition at position 923 in exon 8, predicting an Asn308Ser substitution within the PTP domain, was identified in an unrelated kindred with classical Noonan’s syndrome. No other patients with NL/MGCLS had been evaluated for the PTPN11 mutation.