Metalloprotease-disintegrin ADAM12 is overexpressed and frequently mutated in breast cancer. We report here that ADAM12 expression in cultured mammalian cells is up-regulated by Notch signals. Expression of a constitutively active form of Notch1 in murine fibroblasts, myoblasts, or mammary epithelial cells or activation of the endogenous Notch signaling by co-culture with ligand-expressing cells increases ADAM12 protein and mRNA levels. Up-regulation of ADAM12 expression by Notch requires new transcription, is activated in a CSL-dependent manner, and is abolished upon inhibition of IκB kinase. Expression of a constitutively active Notch1 in NIH3T3 cells increases the stability of Adam12 mRNA. We further show that the microRNA-29 family, which has a predicted conserved site in the 3'-untranslated region of mouse Adam12, plays a critical role in mediating the stimulatory effect of Notch on ADAM12 expression. In human cells, Notch up-regulates the expression of the long form, but not the short form, of ADAM12 containing a divergent 3'-untranslated mRNA region. These studies uncover a novel paradigm in Notch signaling and establish Adam12 as a Notch-related gene.