A comparative analysis of Chk1 phosphorylation levels in the same tumour cell lines growing as xenografts in nu/nu mice (Fig. 4a) or on plastic (Fig. 4b) identified profound differences in the levels of pChk1 (S296), a marker of Chk1 kinase activity, and pChk1 (S317), an ATR phosphorylation site necessary for Chk1 activation, between the two growth systems. Here, ATR is linked to neoplasm.