Abnormalities of the upstream region of the GNAS locus or of genes or transcripts within the GNAS cluster (i.e., STX16, NESP55 and NESPAS), which affect GNAS methylation, are reported in PHP1B.107,108 Genetic testing for patients with the AHO phenotype (i.e., PHP1A/PPHP) should initially involve GNAS sequencing as this reveals coding-region mutations in ~70% of patients, whilst testing for PHP1B may include methylation analysis of the exon 1A differentially methylated region (DMR) and/or evaluation for STX16, NESP55 or NESPAS gene deletions.4,5,107,108. Here, GNAS-AS1 is linked to pseudohypoparathyroidism type 1A.