PIK3CA and small cell carcinoma: Mechanisms of acquired resistance to osimertinib are known to be heterogenous, including EGFR C797X; loss of T790 M; SNV of PIK3CA, KRAS, and BRAF; and amplification of MET. 24, 25In addition to these variants, epithelial‐to‐mesenchymal transition, manifested as small cell carcinoma (SCLC) transformation, is not an infrequent cause of EGFR‐TKI resistance.