Patients with pseudohypoparathyroidism type Ib (PHP-1b) develop resistance toward PTH, leading to hypocalcemia and hyperphosphatemia. PHP-1b is an imprinted human disorder associated with methylation changes at one or several differentially methylated regions at the GNAS locus. This complex locus gives rise to several different transcripts with different patterns of imprinted expression depending on promoter methylation. They can be either coding [Gαs, XLαs, and neuroendocrine secretory protein-55 (NESP55)] or nontranslated (A/B and AS). The paternal AS transcript lies antisense to nesp55.Define the genetic defect in a new family with three patients presenting autosomal dominant PHP-1b.We used methylation analysis, comparative genomic hybridization, and genotyping to characterize the defect. AS expression was studied in two patients and their unaffected mothers.A novel deletion of 18,988 bp that removes NESP55 and a large part of its counterpart GNAS AS intron 4 was discovered. On maternal transmission, this deletion causes loss of A/B methylation without affecting XL/AS imprint. On paternal transmission, there are no methylation anomalies. The deletion creates a cryptic exon contained within AS intron 4, which is expressed from the mutated allele, be it paternal or maternal.This new deletion suggests that NESP55 is an additional imprinting control region that directs A/B methylation in humans. We bring arguments in support of the theory of reciprocal inhibition between the expression of NESP and AS. However, determining whether loss of methylation at the A/B differentially methylated region is a consequence of the loss of NESP expression or of the expression of AS requires additional investigations.