Because abnormal activation of RAS/ERK signalling is frequently observed in human cancers [8] and both DUSP5 and DUSP6 are often up-regulated in tumours and cancer cell lines, which harbour activating mutations in the RAS/ERK pathway [9–13], these enzymes have been presumed to negatively regulate the oncogenic potential of signalling and to be potential tumour suppressors. Here, MAPK1 is linked to cancer.