The p300-kDa N-terminal MLL WT protein fragment is absent in an MLL PTD⁺ primary AML blast sample. Immunoblot analysis was performed as described in “Materials and methods.” Lane 1, Mgc80-3 gastric carcinoma cell line with the MLL PTD gene rearrangement; lane 2, primary AML (UPN 300) with the MLL^PTD/WT genotype; lane 3, primary AML (UPN 003) with MLL^WT/WT genotype; and lane 4, K562 erythroleukemia cell line with the MLL^WT/WT genotype. Arrows indicate the p300-kDa MLL WT posttranslational cleavage N-terminal fragment and the predicted approximate 420-kDa MLL PTD N-terminal cleavage products. Additional bands between p300 and p420 in the MLL PTD⁺ samples may be alternative splicing products or degradation products. Note that, to gain the signals in lane 2, the blot was exposed for a longer period.