THE AUTHORS STATE THAT THE BIOLOGICAL UNIT IS A INDEFINITELY LONG PAIR OF SHEETS (A PROTOFIBRIL). ONE SHEET FORMED BY CHAIN A AND CRYSTALLOGRAPHIC TRANSLATIONS ALONG THE "B" CELL DIMENSION (E.G. X,Y,Z AND X,Y+1,Z). THE SECOND SHEET IS CONSTRUCTED FROM 1-X, Y+1/2,1-Z AND CRYSTALLOGRAPHIC TRANSLATIONS ALONG THE "B" CELL DIMENSION (E.G. 1-X, Y+3/2,1-Z). A SECOND BIOLOGICAL UNIT, SIMILAR TO THE FIRST, IS AN INDEFINITELY LONG PAIR OF SHEETS. ONE SHEET FORMED BY CHAIN B AND CRYSTALLOGRAPHIC TRANSLATIONS ALONG THE "B" CELL DIMENSION (E.G. X,Y,Z AND X,Y+1,Z). THE SECOND SHEET IS CONSTRUCTED FROM -X, Y+1/2,-Z AND CRYSTALLOGRAPHIC TRANSLATIONS ALONG THE "B" CELL DIMENSION (E.G. -X, Y+3/2,-Z).