The monoclonal antibody 2F1 reacts with mouse killer cell lectin-like receptor G1 (KLRG1; formerly known as mouse MAFA), a homodimeric member of the lectin-like type 2 transmembrane receptor family that contain characteristic immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in their cytoplasmic domains. These ITIMs interact with the SH2 domains of protein phosphatases such as SHP-1. The 2F1 antibody stains 30-60% of NK1.1+CD3- splenocytes, and a small fraction of T cells in all mouse strains tested (C57BL/6, BALB/c, 129/J, C3H.SW, AKR/J, SJL). Cell surface expression of KLRG1 is up-regulated by expression of MHC class I molecules. The effect of MHC class I expression is indirect, and can be mediated by interactions with class I-specific Ly49 inhibitory receptors.