In view of the requirement of HOX cofactors, such as PBX proteins, for HOXB7 oncogenic activity,9 we measured the levels of PBX1 and PBX2 proteins in melanoma cell lines, representative of early and advanced stages, showing that PBX2 protein was more abundant than PBX1 (Fig. 1c, bottom). Here, PBX1 is linked to melanoma.