Like JUN and FOS, the Maf transcription factors belong to the AP1 family. Besides their established role in human cancer — overexpression of the large Maf genes promotes the development of multiple myeloma — they can display tumour suppressor-like activity in specific cellular contexts, which is compatible with their physiological role in terminal differentiation. However, their oncogenic activity relies mostly on the acquisition of new biological functions relevant to cell transformation, the most striking characteristic of Maf oncoproteins being their ability to enhance pathological interactions between tumour cells and the stroma.