Other causes of mutation-negative MEN1 syndrome may include the chance co-occurrence of two endocrine tumors without an underlying germline predisposition syndrome or a germline mutation in a gene not commonly screened for as part of a MEN1 panel (e.g., aryl hydrocarbon receptor-interacting protein (AIP OMIM: 605,555) mutations in familial isolated pituitary adenoma) with the co-occurrence of sporadic PHPT [42]. Here, MEN1 is linked to multiple endocrine neoplasia type 1.