MMP2 and fibrosarcoma: We thank Dr Carlos López-Otín and Dr Xose S Puente (University of Oviedo, Spain) for kindly providing pcDNA3-human MMP-2, pCEP-Puro-MMP-2 and pCEP-Puro-mutMMP-2 plasmids, and HT-1080 fibrosarcoma cells conditioned medium.