Clone HBF-719 recognizes the mouse killer cell lectin-like receptor 6 (Ly-49F) antigen, a type II transmembrane natural killer cell receptor belonging to the Ly-49 family. Members of the Ly-49 family are homodimeric C-type lectin receptors, recognizing MHC class I molecules as ligands and include activating and inhibitory receptor molecules. The inhibitory receptor Ly-49F is expressed on subsets of NK cells, NK-T cells, and on a subpopulation of memory T cells in C57BL/6 mice, but not AKR/J, BALB/c, C3H/HeN, DBA/1, or SJL mice. IL-21 selectively down-regulates Ly-49F on mature NK cells.