ctDNA, which is derived from tumor cells, contains tumor-specific gene mutation sites and methylation sites, such as the EGFR gene in non-small cell lung cancer (NSCLC), a BRCA1/2 mutation and methylation in breast cancer, a KRAS gene mutation in colorectal cancer, and a BRAF gene mutation in thyroid cancer. The gene discussed is BRCA1; the disease is neoplasm.