Component of the virion core that undergoes proteolytic processing during the immature virion (IV) to mature virion (MV) transition. Essential for the formation of a structurally normal core (By similarity). Localizes to the virion core. Localizes to the virion core. Expressed in the late phase of the viral replicative cycle. The 25-kDa precursor is cleaved to a mature protein of 17 kDa during virion maturation. Further proteolytic processing is supposed to occur since five more A12L-derived products have been observed (By similarity). Belongs to the chordopoxvirinae A12 family.