Abnormalities of chromosome 11 involving mixed lineage leukemia (MLL) on 11q23 are often seen in acute myeloid leukemia (AML)-M5 or AML-M4. The fusion gene of MLL-PTD and MLL plays a critical role in the pathogenesis of these AML. However, rare chromosome abnormalities have been identified in this type of leukemia. To explore whether there were other MLL gene mutations at M4 and M5, in this study all of the MLL exons were sequenced at cDNA level. 25 patients with de novo AML-M4 or M5 with normal karyotypes excluding M4eo and MLL fusion gene or MLL-PTD were selected, the amplification and direct sequencing analysis of full length MLL gene exons were carried out, then the mutations found were verified at genomic DNA level. Furthermore, the point mutations were tested in normal samples and a larger group of AML patients using the platform of Mass Array. The results showed that high-frequency deletion/insertion and point mutations in RD, PHD, TAD and SET domains of MLL were found, while these alterations in normal samples and other subtypes of AML samples were also verified, and without significant difference (P > 0.05). It is concluded that a variety of deletions/insertions in MLL mRNA and point mutations are respectively alternative splicing of MLL gene at transcriptional level and single nucleotide polymorphism. These alternations together constituted genetic polymorphisms of MLL. Although these variations may not play a direct role in the molecular pathogenesis of AML-M4 or M5, their correlations to clinical treatment and prognosis need to be further explored.