Phosphorylation on tyrosine residues is a key step in signal transduction pathways mediated by membrane proteins. Although it is known that human breast cancer tissue expresses at least 2 MUC1 type 1 membrane proteins (a polymorphic high molecular weight MUC1 glycoprotein that contains a variable number of tandem 20 amino acid repeat units, and the MUC1/Y protein that is not polymorphic and is lacking this repeat array) their function in the development of human breast cancer has remained elusive. Here it is shown that these MUC1 proteins are extensively phosphorylated, that phosphorylation occurs primarily on tyrosine residues and that following phosphorylation the MUC1 proteins may potentially interact with SH2 domain-containing proteins and thereby initiate a signal transduction cascade. As with cytokine receptors, the MUC1 proteins do not harbor intrinsic tyrosine kinase activity yet are tyrosine phosphorylated and the MUC1/Y protein participates in a cell surface heteromeric complex whose formation is mediated by two cytoplasmically located MUC1 cysteine residues. Furthermore, the MUC1/Y protein demonstrates sequence similarity with sequences present in cytokine receptors that are known to be involved in ligand binding. Our results demonstrate that the two MUC1 isoforms are both likely to function in signal transduction pathways and to be intimately linked to the oncogenetic process and suggest that the MUC1/Y protein may act in a similar fashion to cytokine receptors.