Next, we consider the stacking of the hydrogen-bonded layers in guanine. Guanine, one of the purine base pairs which make up deoxyribonucleic acid (DNA), has two known forms which are also polytypes of each other . Each form has a stacking sequence which can be described exclusively using translation vectors. Layers in α guanine are related by applying one vector (Figure ), whilst layers in β guanine are related by applying another vector (Figure B). Combinations using both A and B stacking might be possible but are not observed in currently known crystal forms, suggesting that this combination is a less energetically favourable motif. This may explain why streaking in ED data from guanine is not always observed , given that the origin of streaking in xanthine likely comes from the variation of stacking sequence. However, as in hypoxanthine, streaking in biogenic guanine crystals is observed when crystals are developing . Mature crystals exhibited non-streaked, single-crystal diffraction patterns, pointing to a gradual increase in order as molecules of guanine preferentially oriented along one-dimension arrange into stacked layers. Whilst this variation is seen across individual xanthine particles, it is also possible that defects in guanine and hypoxanthine remain 'hidden' from electron diffraction data as streaking may also not be visible unless the particles of interest are viewed in a direction normal to the layered planes .