FGFR3 and breast cancer: Within this context, it is worth noting that all of the available human BC cell lines are derived from muscle-invasive tumors (with the possible exception of RT4), and it seems likely that muscle-invasive BCs that gave rise to the FGFR inhibitor-resistant, FGFR3 mutant cell lines had progressed beyond the point where mutant FGFR3 was essential to maintain proliferation and/or survival.