A fusion protein (~2250 aa, ~240 kDa) encoded by the MLL/AFF1 fusion gene. This protein is comprised of the N-terminal half of the histone-lysine N-methyltransferase MLL protein, including the AT hook DNA binding domain and the DNA methyltransferase domain, fused to the C-terminal half of the AF4/FMR2 family member 1 protein. [ ]