More invasive phenotypes were also observed using transient transfections of TBX3iso1 or TBX3iso2 into T47D (luminal A; ER+/PR+/HER2−), SKBR3 (HER2‐enriched; ER−/PR−/HER2+), and MDA‐MB‐468 (basal‐like; ER−/PR−/HER2−) breast cancer cell lines, representing different molecular subtypes (supplementary material, Figure S6A–C). Here, ESR1 is linked to breast cancer.