A vary rare acute leukemia that does not express markers considered specific for lineage determination. (e.g. MPO, esterase, cCD3, cCD22, strong CD19, etc.). Typically there will be one or less lineage markers for any lineage. CD34, HLA-DR, and TdT may be expressed, but are not specific for any lineage.
Care should be taken that immunophenotyping panels are extensive, and consideration is given to other entities such as plasmacytoid dendritic precursors, non-hematopoietic tumors, NK-cell precursors, etc. Essentially, a diagnosis of exclusion.
References
Swerdlow SH, Campo E, Harris, NL, Jaffe ES, Pileri SA, Stein H, Thiele J (Eds): WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues (Revised 4th edition). IARC: Lyon 2017