HEY1 and neoplasm: Moreover, we observed a pattern of increased mRNA and protein expression of Hey1 in metastatic cells compared to primary tumor cells and a corresponding decrease in Hes1 expression in cell lines derived from metastatic sites (Fig. 2f) This suggests that Hes and Hey genes have distinct roles in tumorigenesis and that a ‘switch’ from Hes1 to Hey1 expression is associated with the metastatic phenotype.