[An enzyme that catalyzes endonucleolytic cleavage to 5-phosphomononucleotides and 5-phosphooligonucleotide end-products. It has a preference for single-stranded substrates but is active with either ribo- or deoxyribonucleic acids. (From Enzyme Nomenclature, 1992) EC 3.1.30.1. ( MSH )]