Molecular function

histone H4 methyl lysine(20) N-methyltransferase activity (H4-K20 specific), histone H4K20me methylase activity

Catalysis of the reaction: N(6)-methyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-methionine = H+ + N(6),N(6)-dimethyl-L-lysyl(20)-[histone H4] + S-adenosyl-L-homocysteine. This reaction is the addition of a methyl group to the monomethlyated lysine residue at position 20 of histone H4, producing H4K20me2.