2-(1,1-Dimethylethyl)-4-iodo-1,3-dimethoxy-benzene-d6 is labelled 2-(1,1-Dimethylethyl)-4-iodo-1,3-dimethoxy-benzene (D461795) which is used to prepare bis mono- and bicyclic aryl and heteroaryl compounds which inhibit EGF and/or PDGF receptor tyrosine kinase.