Surface expression of CXCR4 and CD44 in leukemia is regulated by the BM microenvironment, while CD34 expression is highly correlated with CD44 but not with CXCR4 in all subsets of spleen and BM, suggesting that each molecule carries a unique function in the interplay between xenograft leukemic cells and their microenvironment. This evidence concerns the gene CD44 and leukemia.