The disorders to be considered here, in contrast to most Mendelian conditions, will be encountered frequently by those working in all branches of medicine and in primary care. The common European definition of a rare disease is given, arbitrarily, as a population frequency of those affected being no greater than 1 in 2,000 (i.e. a prevalence of no greater than 0.05%, or 5 in 10,000). There can be some debate as to which disorders this includes, especially with late-onset disorders or those of reduced penetrance, but in general the distinction is clear enough for practical purposes. Any disorder with a higher prevalence, or a higher incidence at birth, will be regarded as ‘common’.