Costello syndrome is caused by any of at least five different mutations in the "HRAS" gene on chromosome 11. This gene provides instructions for making a protein, H-Ras, that helps control cell growth and division. Mutations that cause Costello syndrome lead to the production of an H-Ras protein that is permanently active. Instead of triggering cell growth in response to particular signals from outside the cell, the overactive protein directs cells to grow and divide constantly. This unchecked cell division may predispose sufferers to the development of benign and malignant tumors. It remains unclear how mutations in "HRAS" cause other features of Costello syndrome, but many of the signs and symptoms may result from cell overgrowth and abnormal cell division.