applying a rotation about [101]/reflection about (101 ̅ ) in Form I flips the layers, leading to twinning. To consider the similarities between polytypes and twins in xanthine, we studied the stacking sequence in the known crystalline (triclinic) form of hypoxanthine, another product of the purine degradation pathway. Hypoxanthine has an intra-layer structure highly similar to that of xanthine, deriving from similar hydrogen-bonding interactions . However, hypoxanthine has different interplanar stacking (Fig ). In its triclinic form, the relationship between adjacent layers is described by a translation combined with a 180° rotation which alternates between each layer about two perpendicular axes (one axis coming out of the page in Figure A, one axis in the plane of the page in Figure ). This stacking sequence is reminiscent of the twinning mechanism observed in xanthine; both offer a way to alter the stacking of hydrogen-bonded layers. The preferred tendency for layers of hypoxanthine to flip suggests a lower associated energy cost than in twinning in the xanthine structure. Interestingly, selected area electron diffraction of hypoxanthine particles at different stages of maturation also exhibited a range of similar patterns . These patterns range from: Hypoxanthine also has a second form , a monoclinic polytype. Although hydrogen bonding arrangements in both forms are essentially identical, there is greater out of plane flexibility in the monoclinic form, resulting in larger canting of the molecules within the layers (Figure .1). Layers in this polytype are related by a translation vector (Figure ) with no rotation.