ESR1 and breast cancer: Tumor receptor subtypes were classified into three groups: Group 1 was comprised of ER+ breast cancer which expressed either ER or PR and lacks HER2 expression; Group 2 was comprised of HER2+ breast cancer which expressed HER2 as determined by IHC and/or FISH with or without expression of ER or PR; and Group 3 was comprised of TNBC which lacked expression of ER, PR, and HER2.