RNA-seq analysis revealed the presence of HER2 L755S mutation in the LapR and NrbR derivatives, but not in P cells...HER2 L755S mutation is acquired upon resistance to lapatinib and neratinib and confers cross-resistance to tucatinib and trastuzumab in HER2- positive breast cancer cell models...On the other hand, the resistant derivatives were cross-resistant to Tuca, both as a single-agent and in combination with T.