In view of the fact that significant populations of CTCs without detectable CK18 are present in both patients and PDX mouse models of different types of cancer, expression of CK18 was examined by CK18-iFISH in both tumor cell lines and enriched CTCs to rule out the possibility that undetectability or absence of CK18 in CTCs is a consequence of the CK18-iFISH methodology. This evidence concerns the gene KRT18 and neoplasm.