The finding that hAG-2 and hAG-3, two unique secreted proteins whose expression is elevated in OR-responsive breast tumours, interact with the same two known extracellular proteins in vitro, C4.4a and dystroglycan, could indicate a role for the hAG proteins in the development and/or progression of breast cancer consistent with the known developmental role of AGR1/2 in Xenopus embryos. This evidence concerns the gene DAG1 and breast cancer.