▁en zym es . ▁macro H 2 A ▁contains ▁a ▁hist one ▁fol d ▁domain ▁and ▁an ▁extra , ▁long ▁C - term inal ▁macro ▁domain ▁which ▁can ▁bind ▁poly - AD P - ri bose . ▁This ▁hist one ▁variant ▁is ▁used ▁in ▁X - in activ ation ▁and ▁trans cription al ▁reg ulation . ▁Struct ures ▁of ▁both ▁domains ▁are ▁available , ▁but ▁the ▁inter - domain ▁linker ▁is ▁too ▁flexible ▁to ▁be ▁cry st all ized . ▁H 2 A . B ▁( B arr ▁body ▁def ic ient ▁variant ) ▁is ▁a ▁rapidly ▁evol ving ▁m amm al ▁specific ▁variant , ▁known ▁for ▁its ▁invol vement ▁in ▁s per mat ogen esis . ▁H 2 A . B ▁has ▁a ▁short ened ▁dock ing ▁domain , ▁which ▁wra ps ▁around ▁a ▁short ▁DNA ▁region . ▁H 2 A . L ▁and ▁H 2 A . P ▁variants ▁are ▁closely ▁related ▁to ▁H 2 A . B , ▁but ▁are ▁less ▁studied . ▁H 2 A . W ▁is ▁a ▁plant ▁specific ▁variant ▁with ▁SP K K ▁mot ifs ▁at ▁the ▁N - ter minus ▁with ▁a ▁put ative ▁minor - gro ove - binding ▁activity . ▁H 2 A . 1 ▁is ▁a ▁m amm al ian ▁test is , ▁o ocy te ▁and ▁z yg ote ▁specific ▁variant . ▁It ▁can ▁prefer entially ▁dim er ize ▁with ▁H 2 B . 1 . ▁It ▁is ▁so ▁far ▁character ized ▁only ▁in ▁mouse , ▁but ▁a ▁similar ▁gene ▁in