Established from a 10-year-old boy with acute monocytic leukemia (AML FAB M5) at diagnosis; cells were described to carry a t(4;11); we found that MV4-11 carries an FLT3 internal tandem duplication; patented cell line. Both granulocyte colony stimulating factor (G-CSF) and granulocyte- macrophage colony stimulating factor (GM-CSF) support the short term and long term growth of MV4-11 cells and act synergistically to stimulate growth of the cells. The cells are dependent upon GM-CSF for long term growth.