While PHP1A is caused by heterozygous mutations involving those GNAS exons that encode Gsα, these inactivating genetic defects on the maternal allele lead to disease only because paternal Gsα expression is, through as‐yet undefined mechanisms, reduced or absent in some tissues, including the proximal renal tubules.(12, 30) The maternal 3‐kb STX16 deletion, which was now identified as the cause of PHP1B in kindred E, leads to loss of methylation at GNAS exon A/B, thereby reducing or abolishing maternal Gsα expression. The gene discussed is STX16; the disease is pseudohypoparathyroidism type 1B.