Nephrogenic rests consist of foci of primitive renal cells, typically
microscopic, that are found within the normal kidney tissue of childre
n with Wilms' tumour. To study the relationship between nephrogenic re
sts and the associated tumours, we screened these lesions for mutation
s in the 11p13 Wilms' tumour suppressor gene, WT1. In two cases in whi
ch the Wilms' tumour contained a somatic WT1 mutation, the nephrogenic
rest had the identical mutation. Nephrogenic rests and Wilms' tumours
are therefore topographically distinct lesions that are clonally deri
ved from an early renal stem cell. Inactivation of WT1 appears to be a
n early genetic event which can lead to the formation of nephrogenic r
ests, enhancing the probability that additional genetic hits will lead
to Wilms' tumour.