Catalytic subunit of RNase HII, an endonuclease that specifically degrades the RNA of RNA:DNA hybrids. Participates in DNA replication, possibly by mediating the removal of lagging-strand Okazaki fragment RNA primers during DNA replication. Mediates the excision of single ribonucleotides from DNA:RNA duplexes (By similarity). Endonucleolytic cleavage to 5'-phosphomonoester. Manganese or magnesium. Binds 1 divalent metal ion per monomer in the absence of substrate. May bind a second metal ion after substrate binding. Belongs to the RNase HII family. Eukaryotic subfamily.