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1540 Background: MEN1 syndrome results from MEN1 gene mutations and is characterized by primary hyperparathyroidism (PHPT), pancreatic endocrine tumors (PET), pituitary adenomas, thymic and other tumors. About 600 different mutations have been reported on Human Gene Mutation Database and 30 of them presented as Familial Isolated Hyperparathyroidism (FIHP); however, no genotype-phenotype correlations have been identified. Methods: We characterized 2 independent families with newly diagnosed MEN1. Mutation analyses were performed in Dept of Genetics Yale University. Diagnostic tests, surgical treatment were implemented. Results: Family 1 has 21 members. 5 presented with early onset of PHPT and no other tumors. 3 ½ parathyroidectomies were performed. A proband had direct sequencing that showed a mutation in Exon 7, consisting of single base insertion in codon 319 (TAC to TTAC)(c1065_1066insT). There were no deaths in the family. Family 2 has 20 members. 7 were tested and showed a mutation in Exon 2, consisting of single base deletion in codon 103 (CTG to _TG)(c417delC). One 1st generation member died from PET at age 79, three 3rd generation members had multifocal metatstatic malignant gastrinomas (ages 32, 39, 41). The proband had total pancreatectomy at age 32. Five members had PHPT, angiofibromas. Three 2nd generation members had thymic carcinomas (ages 20, 54, 60). Conclusions: Phenotypic presentation of MEN1 syndrome may vary with the same mutation suggesting a lack of genotype-phenotype correlation and the effect of modifying factors. On the other hand, these two families—one with a mutation predicted to truncate the menin protein close to its terminus and the other with a truncating mutation early in the coding sequence—have dramatically different severity of the syndrome. Our findings suggest that the exon 7 mutation has a milder effect on protein function resulting in FIHP. The management may be heavily influenced by the predicted clinical phenotype, we are proposing the development of a grading system for MEN1 gene mutations as low-risk and high-risk and to attempt to identify other modifying genetic and environmental factors to improve screening and timing for surgery.