The other possible explanation is that Id-1 expression is tissue specific and has a different role in the development of various tissues that may contribute to different roles in carcinogenesis and in different biologic behaviours of different human cancers (Riechmann et al, 1994; Riechmann and Sablitzky, 1995; Lyden et al, 1999; Norton, 2000). Here, ID1 is linked to cancer.