One of the first studies demonstrating the presence of cancer-initiating stem cells (CICs) in solid tumors revealed that CD44+/CD24-/ESA+ primary breast cancer cells initiate tumorigenesis in nude mice using as few as 200 cells while CD44-/CD24+ cells cannot, even when using up to 1000 fold greater cell numbers [159]. Here, CD24 is linked to breast carcinoma.