A Hippo in the ointment: MST signalling beyond the fly

The regulation of cell cycle and apoptosis is fundamental to the control of cell growth and organism homeostasis. Failure to efficiently regulate these processes often results in the increased cell growth observed in tumours. Accumulation of genetic lesions frequently eliminates these regulatory steps so it is imperative that multiple signalling pathways are employed to ensure that efficient control is maintained. Over the last few years a novel signalling pathway entered the limelight that prevents inappropriate activation of the cell cycle and can elicit apoptosis to limit cell numbers. Denoted the MST/hippo pathway, it is involved in regulating cell number in organism development and tumour progression. Here we aim to review the evidence for a conserved pathway from flies to mammals, and of equal importance to initiate the discussion on the additional cellular and signalling processes that have been adopted by this pathway to achieve further regulation and diversified cellular outcomes in mammals.