One potential explanation may be that although both isoforms may play a role in the induction of FasL expression through production of PGE2, in vivo COX-2 is the isoform that is increased in colon cancer, with a concomitant increase in PGE2 (Rigas et al, 1993; Eberhart et al, 1994). This evidence concerns the gene PTGS2 and malignant colon neoplasm.