Approximately 100 genes undergo genomic imprinting. Mutations in fewer than 10 imprinted genetic loci, including GNAS, are associated with complex human diseases that differ phenotypically based on the parent transmitting the mutation. Besides the ubiquitously expressed Gsα, which is of broad biological importance, GNAS gives rise to an antisense transcript and to several Gsα variants that are transcribed from the nonmethylated parental allele. We previously identified two almost identical GNAS microdeletions extending from exon NESP55 to antisense (AS) exon 3 (delNESP55/delAS3-4). When inherited maternally, both deletions are associated with erasure of all maternal GNAS methylation imprints and autosomal-dominant pseudohypoparathyroidism type Ib, a disorder characterized by parathyroid hormone–resistant hypocalcemia and hyperphosphatemia. As for other imprinting disorders, the mechanisms resulting in abnormal GNAS methylation are largely unknown, in part because of a paucity of suitable animal models. We now showed in mice that deletion of the region equivalent to delNESP55/delAS3-4 on the paternal allele (ΔNesp55^p) leads to healthy animals without Gnas methylation changes. In contrast, mice carrying the deletion on the maternal allele (ΔNesp55^m) showed loss of all maternal Gnas methylation imprints, leading in kidney to increased 1A transcription and decreased Gsα mRNA levels, and to associated hypocalcemia, hyperphosphatemia, and secondary hyperparathyroidism. Besides representing a murine autosomal-dominant pseudohypoparathyroidism type Ib model and one of only few animal models for imprinted human disorders, our findings suggest that the Nesp55 differentially methylated region is an additional principal imprinting control region, which directs Gnas methylation and thereby affects expression of all maternal Gnas-derived transcripts.