In biopsy specimens and cytology specimens, the frequency of EGFR mutation in samples with tumor cellularity < 20% was comparable to specimens with tumor cellularity ≥ 20%, and the low tumor cellularity was not associated with mutation rates of ALK, ROS1, BRAF, KRAS, RET, HER2, CMET, NRAS, and PIK3CA. However, Li et al. found that the frequency of T790M mutation in specimens with tumor cellularity < 20% after TKI drug treatment was significantly lower than that in specimens with tumor cellularity ≥ 20% [7]. The gene discussed is ERBB2; the disease is neoplasm.