Complex sentence: Once such example of the reglatory role that phosphorylation plays is the p53 tumor suppressor protein, the p53 protein is heavily regulated M. Ashcroft, M.H.G. Kubbutat, and K.H. Vousden (1999).
Split into simpler sentences: Once such example of the regulatory role that phosphorylation plays is the p53 tumor suppressor protein. The p53 protein is heavily regulated M. Ashcroft, M.H.G. Kubbutat, and K.H. Vousden (1999).