The pituitary-expressed growth hormone 1 gene was screened for mutation in a group of 74 Spanish children with familial short stature resulting in the identification of a novel Ile179Met missense mutation. Variant and wild-type GH were expressed in insect cells and functional studies were performed. The Ile179Met variant was shown to exhibit a similar degree of resistance to proteolysis as wild-type GH, indicating that the introduction of Met does not cause significant misfolding. Secretion of Ile179Met GH from rat pituitary cells was also similar to that of wild-type. Although receptor binding studies failed to show any difference in binding characteristics, molecular modelling studies suggested that the Ile179Met substitution might perturb interactions between GH and the GH receptor loop containing the hotspot residue Trp169, thereby affecting signal transduction. The Ile179Met variant showed the same ability as wild-type GH to activate a STAT 5-responsive luciferase reporter gene assay. Activation of STAT 5 and the ERK pathway by variant and wild-type GH was studied in 3T3-F442A cells using western blotting with antibodies specific for the phosphorlyated (activated) forms of STAT 5 and ERK. In contrast to its ability to activate STAT 5 normally, activation of ERK by the Ile179Met variant was reduced to only 45% of the level produced by wild-type (4.35 ± 0.66 times basal level of activation for the Ile179Met variant as compared to 9.76 ± 2.52 times basal level of activation produced by wild-type GH; mean ± SEM, n = 5 separate experiments, p<0.05, Student's t-test). Whilst differential effects on the activation of distinct signalling pathways by a mutant receptor agonist is unprecedented, these findings also suggest that the ERK pathway could play a role in mediating the action of GH.