In an attempt to isolate and understand tumorigenic from non-tumorigenic breast cancer cells, Al-Hajj and colleagues [40] used antibodies against CD24 (Heat stable antigen/HAS/BA-1) and CD44 (H-CAM/Pgp-1) to separate heterogeneous cell types in patient-derived xenografts grown in immune-compromised mice. Here, CD44 is linked to breast cancer.