CD44 and neoplasm: To functionally assess the role of tumor-initiating capabilities of the cell populations in the formation of mammary chimeras, 10,000 CD44-enriched or 10,000 CD44-depleted human breast cancer cells were mixed with 50,000 mouse mammary epithelial cells and implanted into the epithelium divested fat pads of three-week-old female athymic nude mice.