CD44 designates a large family of proteins generated from one gene by 
alternative splicing. Variants of CD44 (CD44v) differ from the standard form 
(CD44s) by usage of ten variant exons in various combinations. Some variants 
have been causally related to the metastatic spread of rat tumor cells. In human 
mammary carcinomas and colorectal carcinomas, the expression of CD44v has also 
been correlated with more progressed tumor stages. Moreover, the expression of 
CD44v on mammary and colorectal carcinomas correlates with a bad prognosis for 
patient survival. The biochemical features of these CD44 isoforms that may 
account for both their normal functions and their roles in tumor progression are 
discussed.