When D-ribose-5-phosphate was treated with an ex-tract of mouse spleen, an optically inactive compound X,C3H10O5. was produced. Treatment of X with NaBH4 gave a mixture of the alditols ribitol and xylitol. (See Table 24.2, p. 1193.) Treatment of X with periodic acid produced two molar equivalents of formaldehyde. Suggest a structure for X.