Mucins are a family of heavily glycosylated high molecular weight glycoproteins. A total of 21 mucins have been identified to date. Mucins are well known for its involvement in the protection and lubrication of luminal epithelial surfaces. MUC1, a transmembrane mucin, has been shown to be involved in several signaling pathways, including Ras, beta-catenin, p120 catenin, p53 and estrogen receptor alpha. When MUC1 forms a complex with beta-catenin, it enters the nucleus to activate T-cell factor/leukocyte enhancing factor 1 transcription factors and gene expression. In addition, MUC1 may inhibit cell-cell and cell-stroma interactions and function as a signal transducer, participating in cancer progression. MUC1 is expressed in many types of epithelial cells in gastrointestinal tract, lung, breast, pancreas and genitourinary tract. MUC1 is also detected in activated and unactivated T cells. In some tumors derived from epithelial cells, MUC1 expression is associated with tumor type and invasiveness. MUC1 expression has been correlated with invasive growth of ductal carcinomas (IDC) in the pancreas and cholangiocarcinomas in the liver. MUC2 expression has been associated with the intraductal papillary mucinous tumors of the pancreas, a non-invasive carcinoma. Additionally, MUC1 antibody aids in the prediction of the aggressiveness of carcinomas of the breast, stomach, colon, ampulla of Vater and renal cell carcinoma. Strong correlation has been observed between MUC1 expression and breast cancer progression.