TOPK binds directly to and phosphorylates c-Jun, which consequently activates the transcription of AP-1 target genes, including CCND1 and CDC2. TOPK silencing sensitizes EGFR-TKI-resistant lung cancer cells to gefitinib and increases gefitinib efficacy in preclinical lung adenocarcinoma xenograft models. Here, CCND1 is linked to lung cancer.