The study of the gonads of 8 cases of Drash syndrome (6 ambiguous males, 2 females) and of 2 Frasier syndrome shows that WT1 mutations gives a dysgenetic testis which is the cause of the genital ambiguity observed at birth. By contrast the same mutations have no effect on ovary development giving normal females. However intron mutations in KTS with isoforms imbalance of WT1 proteins cause streak gonads with a female phenotype in XY patients. In consequence WT1 mutations are the cause of a spectrum of male genital malformations associated with glomerulonephritis and tumors. The absence of WT1 protein detection in sertoli cells shown by immunohistochemistry for 3 cases suggests an imprinting effect of the normal WT1 allele promotor rather than a low level of protein production. A caryotype is mandatory for a correct diagnosis.