Heterodimerizes with pms1 to form MutL alpha, a component of the post-replicative DNA mismatch repair system (MMR). 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 mlh1 and pms1 (MutL alpha), and mlh1 and mlh3 (MutL gamma). Forms a ternary complex with MutS alpha (msh2-msh6) or MutS beta (msh2-msh3). Interacts with exo1 (By similarity). Belongs to the DNA mismatch repair MutL/HexB family.