P regulates many aspects of growth, survival and differentiation in cells. The transcription factors of this family are activated by Janus kinase (or 'Just Another Kinase', JAK) and dysregulation of this pathway is frequently observed in primary tumours and leads to increased angiogenesis, enhanced survival of tumours and immunosuppression. Gene knockout studies have provided evidence that STAT proteins are involved in the development and function of the immune system and play a role in maintaining immune tolerance and tumour surveillance.