Several selected peptides of human XRCC4 and mouse Xkr4 were measured by PRM. LC-MS/MS analysis was performed on an EASY-nLC 1200 UHPLC connected to a Q Exactive Plus mass spectrometer through a nanoelectrospray ion source (Thermo Fisher Scientific). Targeted MS/MS scans were acquired by a time-scheduled inclusion list at a resolution of 70,000, an isolation window of 4.0 m/z, a maximum injection time of 2 sec, and a normalized collision energy of 27. Time alignment and relative quantification of the transitions were performed with PinPoint version 1.4 (Thermo Fisher Scientific).