In the previous chapters we examined different aspects of PWS drawing upon anecdotal information, previously published research and systematically collected data from our Cambridge population-based study, to help clarify and illuminate particular issues. Whilst a considerable amount is now known about the syndrome, some of the most crucial aspects still remain elusive. Most striking is whether it is caused by the absence of expression of one or more genes, and which genes. What is clear is that these genes are normally only expressed when inherited from the father, being imprinted when inherited from the mother. At present the SnoRNAs are the main candidates, but until this is fully elucidated, it is not possible to say what the ‘PWS gene(s)’ do and therefore what does not happen when they are not expressed. The second area that remains a considerable mystery is the mechanisms that provide the link between the abnormal gene expression (genotype) and the physical and behavioural phenotype. The crucial issue here is whether all that is characteristic of people with PWS can be explained by one mechanism (and by implication the absence of expression of one gene) or whether more than one underlying process is necessary to explain the full phenotype (and by implication absence of expression of more than one gene).