To assess the antigenic variation of E2 between the subgroup 1.1 C-strain and subgroup 2.1 field isolates, the respective rE2-AD proteins were cross-examined by ELISA with antisera collected from pigs at different time points after immunization with the vaccine C-strain or infection with strain QZ-07 (representing subgroup 2.1). Here, GPR161 is linked to infection.