The present invention describes a newly discovered full-length polynucleotide encoding an SH2 domain-containing adapter protein, called human MIST, cloned, isolated and identified from a human spleen cDNA library. Also described are the MIST polypeptide sequence, expression vectors, host cells, agonists, antagonists, antisense molecules, and antibodies related to the polynucleotide and/or polypeptide of the present invention. Novel splice variant forms of human MIST are provided. Methods for screening for modulators, particularly inhibitors, of the MIST protein and use of the human MIST polynucleotide and polypeptide for therapeutics and diagnostics are described.