A missense mutation, C422F, was identified in the intracellular domain of GH receptor (GHR) in a Japanese short boy. Although this mutation was previously reported in a patient with GH insensitivity syndrome (GHIS), it has not been clear whether this mutation causes GH insensitivity. To clarify the effect of this mutation on GH signal transduction, mutant GHR was expressed in CHO cells, and its functional properties were investigated. There were no significant differences in GH-induced tyrosine phosphorylation of STAT5b (signal transducer and activator of transcription) between wild-type GHR (GHR-wt)- and mutant GHR (GHR-C422F)-expressing cells. Moreover, STAT5-mediated transcriptional activation of GHR-C422F-expressing cells was comparable to that of GHR-wt-expressing cells. These findings indicated that the C422F mutation of GHR affected neither GH-induced tyrosine phosphorylation nor the transcriptional activation of STAT5. In addition, the analysis of genotypes and phenotypes of his family revealed that body heights of family members with heterozygous or homozygous C422F mutations were all within normal ranges, with the single exception of the proband. These in vitro and in vivo results indicate that the C422F missense mutation of GHR is a polymorphism that does not result in GHIS.