This invention provides methods of treating, reducing the incidence of, and inhibiting metastasis formation of carcinomas, sarcomas, Epstein-Barr virus-induced malignancies, B cell proliferative disorders, and mast cell activation disorders, comprising administering to a subject a compound that inhibits an interaction of a first protein and an immunoreceptor tyrosine-based activation motif (ITAM) of a second protein, and screening methods for identifying ITAM-inhibitory compounds and peptides. This invention also provides peptides that inhibit signaling by ITAMs.