Transactivation activity of the androgen receptor (AR) is induced by the binding of an androgen to its ligand-binding domain (LBD). The tertiary architecture of the AR LBD, in common with other steroid/nuclear receptors, is a sandwich of 12 alpha-helices (H). We have encountered a missense substitution, M807T, which was associated with partially defective androgen binding in a 46,XY infant with ambiguous genitalia. In contrast, two other substitutions in the same residue 807 to valine and arginine, resulted in almost total abrogation of androgen-binding and complete androgen insensitivity syndrome in two unrelated individuals. We recreated these substitutions in residue 807 and observed that disruption of ligand-binding and transactivation activities was total for M807R and partial for M807V, while the least-affected was M807T. Modelling of the AR LBD indicate that van der Waal interactions between residue 807 (H8) to H9 and H10 were severely disrupted for the arginine mutant, but relatively preserved for the threonine and valine mutants. However, there was a subtle difference between these two variants in that M807T, but not M807V, improved van der Waal contacts with another residue L859 in H10, suggesting the importance of interactions between M807 and L859 for LBD stability. Atomic distances of M807 (H8) to L859 (H10) in corresponding residues of the distantly related ER alpha, RXR alpha, PPAR gamma and VDR LBD are highly conserved and almost invariant, suggesting that H8/H10 interactions are critical for LBD stability in other members of the steroid/nuclear receptor superfamily.