PTEN and multiple endocrine neoplasia type 1: Multiple Endocrine Neoplasia type 1 (MEN1) syndrome,3, 4, 5, 6 Multiple Endocrine Neoplasia type 4 (MEN4, caused by CDKN1B)7, 8 and Cowden syndrome (PTEN)9 have been associated with both NET and breast cancer in case reports or case series.