Objective: The GNAS gene encodes the α-subunit of stimulatory G proteins, which play a crucial role in intracellular signal transduction of peptide and neurotransmitter receptors. In addition to transcript variants that differ in their first exon due to different promoters, there are two long (Gsα-L) and two short (Gsα-S) splice variants, created by alternative splicing. Heterozygous inactivating maternally inherited mutations of GNAS lead to a phenotype in which Albright hereditary osteodystrophy is associated with pseudohypoparathyroidism type Ia. Methods and Results: The GNAS gene of a 10-yr-old girl with brachymetacarpia, mental retardation, normocalcemic pseudohypoparathyroidism, and hypothyroidism was investigated. We found a heterozygous insertion of an adenosine in exon 3 altering codon 85 and leading to a frame shift inducing a stop codon in exon 4. Molecular studies of cDNA from blood RNA demonstrated normal, biallelic expression expression of Gsα-S transcripts, whereas expression of Gsα-L transcripts from the maternal allele was reduced. Immunoblot analysis revealed a reduced Gsα-L protein level to about 50%, whereas the protein level of Gsα-S was unaltered. Furthermore, the Gsα protein activity in erythrocyte membranes was diminished to about 75% of normal. Both the reduced activity and the mutation were also found in the mother and the affected younger brother. Conclusion: This report demonstrates the first evidence for a pathogenic mutation in exon 3 of the GNAS gene. The mutation is associated with a phenotype of Albright hereditary osteodystrophy and pseudohypoparathyroidism type Ia due to selective deficiency of Gsα-L and a partial reduction of Gsα activity.