Described herein is a novel gene and its product, WIP, which associates with WASP. The subject invention relates to the isolated WIP gene or cDNA and transgenic mammals that have the WIP gene disrupted in their genome. Also the subject of this invention are methods of treating conditions or diseases in which WIP and/or WASP DNA or protein is deficient and/or defective, for example, mutated or altered, such that an individual is adversely affected. Also described are methods of altering or regulating WIP and its functions in a mammal or in a cell of a mammal, for example in a lymphocyte. A further subject of this invention is an assay to identify drugs which alter the activity of WIP or expression of WIP DNA.