▁be ▁modified ▁by ▁various ▁post - trans l ational ▁modifications ▁to ▁alter ▁chrom atin ▁pack ing ▁( H ist one ▁modification ). ▁Most ▁of ▁the ▁modifications ▁occur ▁on ▁the ▁hist one ▁tail . ▁The ▁consequences ▁in ▁terms ▁of ▁chrom atin ▁access ibility ▁and ▁comp action ▁depend ▁both ▁on ▁the ▁am ino - ac id ▁that ▁is ▁modified ▁and ▁the ▁type ▁of ▁modification . ▁For ▁example , ▁Hist one ▁ac et yl ation ▁results ▁in ▁lo osen ing ▁and ▁increased ▁access ibility ▁of ▁chrom atin ▁for ▁rep lication ▁and ▁trans cription . ▁L ys ine ▁tri - m eth yl ation ▁can ▁either ▁be ▁correl ated ▁with ▁trans cription al ▁activity ▁( tri - m eth yl ation ▁of ▁hist one ▁H 3 ▁L ys ine ▁ 4 ) ▁or ▁trans cription al ▁repr ession ▁and ▁chrom atin ▁comp action ▁( tri - m eth yl ation ▁of ▁hist one ▁H 3 ▁L ys ine ▁ 9 ▁or ▁ 2 7 ). ▁Several ▁studies ▁suggested ▁that ▁different ▁modifications ▁could ▁occur ▁simultaneously . ▁For ▁example , ▁it ▁was ▁proposed ▁that ▁a ▁b ivalent ▁structure ▁( with ▁tri - m eth yl ation ▁of ▁both ▁L ys ine ▁ 4 ▁and ▁ 2 7 ▁on ▁hist one ▁H 3 ) ▁was ▁involved ▁in ▁m amm al ian ▁early ▁development . ▁ ▁Poly comb - group ▁prote ins ▁play ▁a ▁role ▁in ▁reg ulating ▁gen es ▁through ▁mod ulation ▁of ▁chrom atin ▁structure . ▁ ▁For ▁additional ▁information , ▁see ▁Hist one ▁modifications ▁in ▁chrom atin ▁reg