The T and B cell antigen receptors are associated with transmembrane molecules with immunoreceptor tyrosine-based activation motifs (ITAMs) in their cytoplasmic domains. These tyrosine-containing motifs are sites of tyrosine phosphorylation and of association with tyrosine kinases and other phosphotyrosine-binding moieties involved in receptor signaling. Related motifs with opposing effects are immunoreceptor tyrosine-based inhibitory motifs (ITIMs), which recruit phosphatases to the receptor site that remove the phosphate groups added by tyrosine kinases.