Component of the post-replicative DNA mismatch repair system (MMR). Heterodimerizes with mlh1 to form MutL alpha. DNA repair is initiated by MutS alpha (msh2-msh6) or MutS beta (msh2-msh3) binding to a dsDNA mismatch, then MutL alpha is recruited to the heteroduplex. Assembly of the MutL-MutS-heteroduplex ternary complex in presence of rfc and pcna is sufficient to activate endonuclease activity of pms1. It introduces single-strand breaks near the mismatch and thus generates new entry points for the exonuclease exo1 to degrade the strand containing the mismatch (By similarity). Heterodimer of pms1 and mlh1 (MutL alpha). Forms a ternary complex with MutS alpha (msh2-msh6) or MutS beta (msh2-msh3) (By similarity). Belongs to the DNA mismatch repair MutL/HexB family.