Some authorities view the history of science as a sort of saltatory process in which periods of modest gain and of plodding ‘normal science’ are interrupted by dramatic leaps forward and episodes of ‘revolution’ (Kuhn, 1962). If this is so then genetics has, for the past several years, been in a phase of remarkably sustained and continuous revolution. The advent of the ‘new genetics' of recombinant DNA has resulted in new discoveries occurring at a breath-taking pace, many of which have important clinical implications. Recent findings of psychiatric relevance have included the localisation of the gene for Huntington's chorea on the short arm of chromosome 4 (Gusella et al, 1983) and the use of DNA probes in predictive testing (Harper, 1986). Advances have been achieved in the understanding of the molecular biology of Alzheimer's disease, and at least some of the familial forms of the condition appear to be linked to a gene on chromosome 21 (St George-Hyslop et al, 1987). However, perhaps the most exciting development for most psychiatrists has been the report (Egeland et al, 1987) of a major gene for manic-depressive illness linked to a marker on the short arm of chromosome 11. Could this signal the leap of biological psychiatry into a revolutionary phase? It is perhaps appropriate before attempting to answer this that we give some consideration to the recent historical background.