We previously reported that MEKK1 signaling is required for expression of proteases that are transcriptionally controlled by AP-1 and promote tumor invasiveness (31), so our discovery that MEKK1 controls chemokine gene expression in breast stroma cells strongly supports a model of the breast tumor microenvironment in which MEKK1-dependent gene expression in stroma cells can promote breast cancer cell migration. Here, MAP3K1 is linked to breast cancer.