N(6)-methyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-methionine <=> N(6),N(6)-dimethyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-homocysteine + H(+)
n(6)-methyl-l-lysyl(20)-[histone h4] + s-adenosyl-l-methionine <=> n(6),n(6)-dimethyl-l-lysyl(20)-[histone h4] + s-adenosyl-l-homocysteine + h(+)