). ▁Well ▁studied ▁species ▁specific ▁variants ▁include ▁H 3 . 1 , ▁H 3 . 2 , ▁T S ▁H 3 . 4 ▁( m amm als ), ▁H 3 . 5 ▁( h omin ids ), ▁H 3 . Y ▁( prim ates ). ▁Ex cept ▁for ▁c en H 3 ▁hist one , ▁H 3 ▁variants ▁are ▁highly ▁sequence ▁conser ved ▁differ ing ▁only ▁by ▁a ▁few ▁am ino ▁ac ids . ▁Hist one ▁H 3 . 3 ▁has ▁been ▁found ▁to ▁play ▁an ▁important ▁role ▁in ▁maintain ing ▁gen ome ▁integrity ▁during ▁the ▁m amm al ian ▁development . ▁ ▁Vari ants ▁of ▁hist one ▁H 4 ▁ ▁Hist one ▁H 4 ▁ ▁is ▁one ▁of ▁the ▁slow est ▁evol ving ▁prote ins ▁with ▁no ▁functional ▁variants ▁in ▁the ▁majority ▁of ▁species . ▁The ▁reason ▁for ▁a ▁lack ▁of ▁sequence ▁variants ▁remains ▁unclear . ▁Try pan os oma ▁are ▁known ▁to ▁have ▁a ▁variant ▁of ▁H 4 ▁named ▁H 4 . V . ▁In ▁D ros oph ila ▁there ▁are ▁H 4 ▁replacement ▁gen es ▁that ▁are ▁const itut ively ▁expressed ▁throughout ▁the ▁cell ▁cycle ▁that ▁encode ▁prote ins ▁that ▁are ▁identical ▁in ▁sequence ▁to ▁the ▁major ▁H 4 . ▁ ▁Vari ants ▁of ▁hist one ▁H 2 A ▁ ▁Hist one ▁H 2 A ▁has ▁the ▁highest ▁number ▁of ▁known ▁variants , ▁some ▁of ▁which ▁are ▁relatively ▁well ▁character ized . ▁H 2 A . X ▁is ▁the ▁most ▁common ▁H 2 A ▁variant , ▁with ▁the ▁defining ▁sequence