text: The 3-dimensional structure of micrococcal nuclease (then called Staphyloccal nuclease) was solved very early in the history of protein crystallography, in 1969, deposited as now-obsolete Protein Data Bank file 1SNS. Higher-resolution, more recent crystal structures are available for the apo form as Protein Data Bank file 1SNO: [http://www.rcsb.org/pdb/explore/explore.do?structureId=1SNO] and for the thymidine-diphosphate-inhibited form as Protein Data Bank file 3H6M: [http://www.rcsb.org/pdb/explore/explore.do?structureId=3H6M] or 1SNC: [http://www.rcsb.org/pdb/explore/explore.do?structureId=1SNC]. As seen in the ribbon diagram above, the nuclease molecule has 3 long alpha helices and a 5-stranded, barrel-shaped beta sheet, in an arrangement known as the OB-fold (for oligonucleotide-binding fold) as classified in the SCOP database.