GNAS and pseudohypoparathyroidism type 1B: Most cases of pseudohypoparathyroidism type 1B (PHP1B) are sporadic, whereas autosomal dominant PHP is less common and often associated with maternal derived mutations in STX16. Both forms of PHP1B are associated with abnormal methylation (both abnormal gain and loss of methylation) at differentially methylated regions (DMR) in GNAS exons.