▁data ▁are ▁accum ulating ▁about ▁the ▁roles ▁of ▁diverse ▁hist one ▁variants ▁highlight ing ▁the ▁functional ▁links ▁between ▁variants ▁and ▁the ▁del icate ▁reg ulation ▁of ▁organ ism ▁development . ▁ ▁Hist one ▁variants ▁nom en cl ature ▁ ▁D ifferent ▁names ▁histor ically ▁assigned ▁to ▁hom olog ous ▁prote ins ▁in ▁different ▁species ▁comp licate ▁the ▁nom en cl ature ▁of ▁hist one ▁variants . ▁A ▁recently ▁suggested ▁un ified ▁nom en cl ature ▁of ▁hist one ▁variants ▁follows ▁ph y log eny - based ▁approach ▁to ▁naming ▁the ▁variants . ▁According ▁to ▁this ▁nom en cl ature , ▁letter ▁suffix es ▁or ▁prefix es ▁are ▁mainly ▁used ▁to ▁denote ▁struct ur ally ▁distinct ▁mon oph y let ic ▁cl ades ▁of ▁a ▁hist one ▁family ▁( e . g . ▁H 2 A . Z , ▁H 2 B . W , ▁sub H 2 B ). ▁Number ▁suffix es ▁are ▁assumed ▁to ▁be ▁species - specific ▁( e . g . ▁H 1 . 1 ), ▁but ▁are ▁encourag ed ▁to ▁be ▁used ▁consist ently ▁between ▁species ▁where ▁unique ▁orth ologies ▁are ▁clear . ▁However , ▁due ▁to ▁historical ▁reasons ▁naming ▁of ▁certain ▁variants ▁may ▁still ▁devi ate ▁from ▁these ▁rules . ▁ ▁Vari ants ▁of ▁hist one ▁H 3 ▁ ▁Through out ▁e uk ary otes ▁the ▁most ▁common ▁hist one ▁H 3 ▁variants ▁are ▁H 3 . 3 ▁and ▁cent rom er ic ▁H 3 ▁variant ▁( cen H 3 , ▁called ▁also ▁C EN PA ▁in ▁humans