In this assay, cells are transiently transfected to express oncoproteins and are left to grow for three weeks. Untransfected MCF7 cells displaying an epithelio?��d phenotype are responsive to contact inhibition and show very few, if any, foci after three weeks of culture. The transient transfection of Prep1a and Pbx1 expression vectors in control clones did not enhance foci formation . In contrast, transfecting Hoxa1WT or Hoxa1I-V together with the cofactors resulted in the appearance of numerous foci . Most significantly, when the hexapeptide mutant was cotransfected with the cofactors, a small amount of foci was observed, which was not distinguishable from the situation where only the cofactors were expressed. This assay therefore shows that the Hoxa1WM-AA protein has lost the ability to relieve the cells from their contact inhibition. This again supports that the hexapeptide mutation suppresses the oncogenic potential of Hoxa1. While a continuously increasing number of studies Kinase Inhibitor Library report correlations between Hox genes misexpression and several types of cancers, only a few Hox genes have been identified to actually impact on cancer progression, as genuine oncogenes or tumor suppressors . Hoxa1 has been reported to be abnormally expressed in breast carcinomas and to act as a mammary oncogene . Like many other Hox proteins, Hoxa1 can interact with the TALE homeoproteins Pbx. This interaction relies on a hexapeptidic motif of Hoxa1. It has indeed been demonstrated that substituting two amino acids in this hexapeptide motif abrogated the formation of Hoxa1-Pbx1a complexes on cognate target DNA sequences . Further, we have previously shown that disrupting the Hoxa1-Pbx interaction severely impaired its developmental activity. Indeed, by substituting these two amino acids critically involved in the docking to Pbx, we generated knockin mice which phenocopied the Hoxa1 knockout, suggesting that the Hoxa1-Pbx partnership is crucial to the Hoxa1 function. Here, we addressed the importance of the hexapeptide integrity for the oncogenic potential of Hoxa1. We demonstrate that the Hoxa1WM-AA hexapeptide mutant lost its ability to stimulate cell proliferation, anchorage-independent cell growth and loss of contact inhibition.