Endoribonuclease that cleaves single-stranded RNAs at 5' of uridylates and releases a product with a 2',3'-cyclic phosphate at the 3'-end. The UU and GU sites are more efficiently cleaved than CU and AU sites. ribonucleotidyl-uridine-RNA = a 3'-end 2',3'-cyclophospho-ribonucleotide-RNA + a 5'-end dephospho-uridine-RNA Monomer. Placental-specific, but also associated with various malignant neoplasms. It has been suggested that the active SMB domain may be permitted considerable disulfide bond heterogeneity or variability, thus two alternate disulfide patterns based on 3D structures are described with 1 disulfide bond conserved in both. Belongs to the ENDOU family. Was originally (PubMed:2350438) thought to be a serine protease. However, PubMed:18936097 showed it is not the case.