MUC4 and breast cancer: A further proposed mechanism for Herceptin resistance is the physical blockade or masking of the HER2 receptor18, for example, the MUC4 molecule with its extended carbohydrate structure appears to serve as a barrier for biomolecular interactions in the extracellular environment19, 20 and over-expression of MUC4 in the Herceptin resistant breast cancer cell line JIMT-1 has been shown to lead to diminished Herceptin binding and isolation of the receptor from its normal interaction and activation partner21.