The classic hamartoma syndromes and the related conditions discussed in this chapter show varying degrees of phenotypic and genetic overlap. Knowledge of the susceptibility genes underlying their phenotypes has provided additional information for the classification of these syndromes. Germline PTEN mutations appear to cause both CS and BRR. These two syndromes are therefore likely to be different manifestations of a single disease with variable expression. It has been suggested that PTEN mutation-positive CS and BRR should be grouped as a single entity for clinical purposes and classified as the 'PTEN hamartoma-tumour syndrome' . Germline PTEN mutations are unlikely to cause JPS. However, germline SMAD4 mutation, especially a well-described 4-bp deletion, can be used to confirm a clinical diagnosis of JPS. Like the hamartoma syndromes, the lentiginoses also show substantial clinical overlap; it remains to be seen whether this is reflected in the molecular pathways that are involved in the pathogenesis of these syndromes. Given that the degree and type of cancer susceptibility between the hamartoma and lentiginosis syndromes is different, clarification of these syndromes at the molecular level is predicted to allow directed cancer surveillance. The molecular story is still evolving with regards to aspects of genetic heterogeneity, signalling pathways and the manner in which these hamartoma genes function in the development of their respective syndromes. It is likely that in all of these syndromes, tumours develop against a background created by loss of the growth-suppressive function of their susceptibility gene via mechanisms including disruption of the cell cycle and the activation of anti-apoptotic pathways.