When the BMJ last published an editorial on adult autosomal dominant polycystic kidney disease1 there was no chromosomal assignment for the disorder. Two have since been recognised. The first, designated PKD-1,2 has been localised to a narrow region of about half a million base pairs on the short arm of chromosome 16, but the specific gene has not yet been discovered. A second locus was postulated in 1988,3 and in the past few months two groups have independently confirmed that a second locus exists on chromosome 4q13-q23.^4,5 What proportion of cases that this new locus accounts for and whether other loci exist are unknown.

The discovery of this second locus will improve the accuracy of presymptomatic …