Homozygote and heterozygote null mice are lymphopenic, resulting from diminished homeostatic proliferation and impaired T cell and B cell survival. Mice homozygous for a gene-trapped allele exhibit enhanced neointimal thickening in response to arterial injury, increased angiogenesis in matrigel plugs and aortic ring cultures, and increased neovascularization of tumor implants, and decreased B-cell and increased T-cell population percentages.