ABCG2 and neoplasm: Moreover, ABCG2 expression is high in side-population cells of pancreatic and other tumour cell lines, as well as in primary tumour samples (Hirschmann-Jax et al, 2004; Seigel et al, 2005; Haraguchi et al, 2006; Shi et al, 2008; Sung et al, 2008; Dou et al, 2009; Wang et al, 2009; Yajima et al, 2009), in which it is likely to have a key role in xenobiotic protection, and acts as a marker of side-population cells.