C4A and breast cancer: Based on this method, thirteen peptides were identified their amino acid sequences, and five of these peptides were used to construct a biological age model (fragment of apolipoprotein A-I (m/z 2883.99), fibrinogen alpha chain (m/z 3060.13), complement C3 (m/z 2190.59), complement C4-A (m/z 1898.21), and breast cancer type 2 susceptibility protein (m/z 1607.84)).