Various groups have attempted to define basal-like breast cancers using panels of immunohistochemical markers and microarray-based expression profiling [28], including (1) the triple-negative immunophenotype (absence of ER, PGR, and HER2 expression); (2) the expression of one or more high-molecular-weight/basal cytokeratins (CK5/6, CK14, and CK17); (3) the absence of ER and HER2 expression combined with CK5/6 and/or EGFR expression [26], and (4) the absence of ER, PGR, and HER2 expression in combination with CK5/6 and/or EGFR expression. The gene discussed is ESR1; the disease is breast cancer.