ERBB2 and cancer: In estrogen receptor (ER)-positive cancers, alterations in several genes including ER (ESR1)2,3, neurofibromin (NF1)4,5 and v-erb-b2 avian erythroblastic leukemia viral oncogene homolog 2 (ERBB2, or HER2)6 are common as tumors develop resistance to antiestrogen therapy.