Mast cells are major effector cells of allergic and inflammatory disease. Thus understanding mechanisms of mast cell biology are of great interest to cell biology research. This study aimed to identify the expression and function of a newly identified gene family, the MS4A family, in human mast cells. This study identified 8 gene variants expressed in mast cells including 2 novel variants. All of the expressed genes were cloned and GFP and adenoviral constructs were generated. Quantitative RT-PCR demonstrated that all expressed gene variants were differentially regulated by mast cell stimulation with IgE, IgE/anti-lgE and stem cell factor (SCF) suggesting roles in mast cell biology. Transfections demonstrated that most proteins were trafficked to the cytoplasmic membrane, but some were trafficked to the nuclear membrane. This intracellular localisation of the MS4A family may be critical for their function. This was exemplified in this study by the function of a novel truncation (MS4A2trunc) of MS4A 2 (the beta chain of FceRI). MS4A2trunc was trafficked to the nuclear membrane, whereas MS4A 2 was trafficked to the cytoplasmic membrane. Overexpression of MS4A 2 using adenoviral transduction had no apparent effect on mast cell survival or proliferation. However, overexpression of MS4A2trunc profoundly inhibited mast cell proliferation and induced apoptosis via G2 phase cell cycle arrest. The removal of SCF from mast cell cultures upregulated the expression of MS4A2trunc. In addition, the slowly replicating primary human lung mast cells, and the mast cell line LAD-2 expressed MS4A2trunc. However, I was unable to demonstrate expression of MS4A2trunc in the rapidly replicating c-KIT gain-of-function mutated mast cell line HMC-1. Thus loss of expression of MS4A2trunc may be an important step in the development of mast cell neoplasia. This study has identified an entirely novel function for the MS4A 2 gene and has opened numerous avenues for future research on the MS4A family.