HLCO-75, established from pleural effusion at disease progression after osimertinib treatment was genotyped and found to gain ERBB2 V777L mutation in exon 20 in addition to original EGFR L858R mutation. Marked growth-inhibitory effect by poziotinib, a HER2 tyrosine kinase inhibitor with IC50 value 6 nM was observed whereas not by TAS6417, another EGFR/HER2 exon 20 inhibitor, suggesting the potential of organoid culture for decision-making in cancer precision medicine.