Two years after the first mutation on exon 7 in the carboxy-terminal part of the hinge domain (D) was reported (Behr and Loos 1992), we have identified the second mutation on exon 7 in patients with GRTH. Interestingly, our mutation it is not located in the two previously described "hot spot regions", but instead very close to the hinge domain (D) of the receptor protein that is essential for the function of the hormone binding domain (E) (Lin et al., 1991). Confirming the observation that the majority of single base substitutions causing human genetic diseases or DNA polymorphisms follow the hot spot mutation rule of CG to TG and CG to CA transition (Barker et al., 1984), an additional CpG dinucleotide transition has been identified.