Over 100 mutations have so far been published in the androgen receptor gene (AR) in patients with different degrees of undervirilisation. The AR gene consists of 8 exons, exons 2-3 code for the DNA binding domain and exons 4-8 (codon 628-919) for the steroid binding domain (codon 667-919). Only four mutations have been published in the 5' end of exon 4, codons 628-667, that is in two PAIS cases and two prostate tumours. In a set of phenotypically normal controls we observed one mutation in exon 4, codon 645, in a 15 year old boy. He has a history of Wilms' tumour but no undervirilisation and without a family history with intersex. Exon 4 was amplified from constitutional DNA and subject to DGGE with 35-65% of denaturants. Aberrant fragments were subject to a new PCR and direct sequencing was performed. This mutation changed Ala645-->Asp due to change of GCT-->GAT This was not detected in 108 normal chromosomes. Interestingly, the same mutation was recently reported in one PAIS case. This phenotypic discrepancy calls for functional studies of this region. This is the first case of a normal phenotype with an amino acid alteration in the AR gene.