Epithelial cells of the mammary gland have the ability to proliferate, differentiate and undergo apoptosis. A murine model system for studying mammary epithelial cell behavior is the HC11 cell line, which has some stem and/or progenitor cell characteristics. HC11 cells can be maintained, in response to special treatment conditions, as undifferentiated cells, competent cells capable of responding to lactogenic hormones, or they can be induced to differentiate. Based on data obtained from a gene array, the aim of this project is to investigate differentially expressed genes in HC11 cells, which might be stem/progenitor cell marker candidates and determine their functions in HC11 cells. Polymerase chain reaction (PCR) experiments revealed that Lgals1 and Ran were upregulated in undifferentiated compared to competent and induced HC11 cells. These findings were confirmed and quantified in this project with real-time PCR experiments. Western Blot experiments confirmed the upregulation of galectin-1 (Gal-1), the Lgals1 gene product, in undifferentiated compared to competent, induced and differentiated HC11 cells. Fluorescent activated cells sorting (FACS) analysis of undifferentiated HC11 cells revealed that 60.85% of the gated HC11 cells expressed stem cell antigen 1 (Sca1), 90.12% of the cells expressed Gal-1 and 60.77% of the gated HC11 cells expressed Gal-1 and Sca1. Galectin-1 positively influences cell proliferation and migration of certain cell types. Knockdown of Lgals1 in HC11 cells followed by functional assays measuring proliferation or migration might give further insight into how undifferentiated cells are regulated. In this study it has been shown that galectin-1 (Gal-1), the Lgals1 gene product, is upregulated in undifferentiated compared to competent, induced and differentiated HC11 cells, functional assays might reveal an important function of galectin-1 in HC11 cells, as well as in stem/progenitor cells of the mammary gland and FACS experiments revealed that galectin-1 might be used to enrich stem/progenitor cell populations of the mammary gland.