(en'dō-nū'klē-ās sĕ-rā'shē-ă mar-ses'ens), A nuclease (a nucleate oligonucleotidohydrolase) that forms oligonucleotides ending in 5'-phosphates from RNA and DNA; hydrolyzes both double-stranded and single-stranded polynucleic acids. Synonym(s): nucleate endonuclease