Molecular markers for lung and colorectal carcinomas and methods of using them in blood sample assays are disclosed. The method comprises measuring the expression of the markers in a blood sample from a subject for detecting the presence and/or severity of lung and/or colorectal cancer, and for monitoring and/or assessing the prognosis of the subject's response to a cancer therapy. Also disclosed are kits for detecting, diagnosing, and/or monitoring lung or colorectal carcinomas.