When DNA methylation of APC, HOXA9, RARβ2, and RASSF1A was evaluated by QMSP in lung cancer, it was found that HOXA9 and RASSF1A had higher methylation levels in small‐cell lung cancer than in NSCLC, with HOXA9 methylation levels displaying a sensitivity of 63.8%, and RASSF1A displaying a specificity of 96.2% for small‐cell lung cancer detection in ctDNA [67]. The gene discussed is HOXA9; the disease is lung cancer.