The data related to the patient described in the present study at least suggest that mutations in FGFR3 cause ACH but do not influence the effects of the duplication of GRB10 on prenatal onset growth delay in SRS. The results of our study also suggest that phenotypes are rarely “simple” or directly related to specific gene defects and that combinations of uncommon, rare and exceptional molecular defects, which can be explored and used in diagnoses, may explain the so-called variability observed in the expression of dominant traits. The gene discussed is GRB10; the disease is achondroplasia.