The natural cytotoxicity receptor NKp44, expressed on activated NK cells, binds a ligand, NKp44L, which is expressed at the surface of tumor cells. NKp44L is produced from an alternate transcriptional splice variant of the MLL5 gene, as a truncated MLL5 protein with a unique C-terminal sequence. MLL5 is an intracellular, nuclear protein. NKp44L is found in the cytoplasm and at the cell surface. The mAb 7.1, specific for NKp44L, blocks the killing of NKp44L⁺ target cells by NKp44⁺ NK cells.