RET and multiple endocrine neoplasia: Although pHPT mostly presents sporadically, some mutations have been linked to pHPT in young subjects, including multiple endocrine neoplasia (MEN) syndrome, either type 1 (caused by a mutation in MEN1) or type 2a (caused by a mutation in RET), and hyperparathyroidism-jaw tumor syndrome (caused by a mutation in HRPT2).