Cross-talk in the mammary gland Macrophages engulf damaged and dead cells to clear infection, but they also participate in tissue regeneration. Chakrabarti et al. expand the macrophage repertoire for mammary gland development (see the Perspective by Kannan and Eaves). Mammary gland stem cells secrete the Notch ligand Dll1 and activate Notch signaling, which promotes survival of adjacent macrophages. This stimulates production of Wnt ligands, which signal back to the mammary gland stem cells. This cross-talk plays an important role in coordinating mammary gland development, tissue homeostasis, and, not least, breast cancer. Science , this issue p. eaan4153 ; see also p. 1401