PTH and Hypocalcemia: For example, mutations in PTH cause a lack of functional PTH protein (3), mutations in the parathyroid-specific transcription factor glial cells missing B (GCMB) result in defective parathyroid gland development (4, 5), and gain-of-function mutations in the calcium-sensing receptor (CaSR) may suppress parathyroid gland function in association with hypocalcaemia (6).