Neurofibromatosis type I is caused by a mutation on chromosome 17 encoding a cytoplasmic protein known as neurofibromin. This protein is a tumor suppressor and therefore serves as a signal regulator of cell proliferation and differentiation. A dysfunction or lack of neurofibromin can affect regulation, and cause uncontrolled cell proliferation, leading to the tumors (neurofibromas) that characterize NF1. The neurofibromas caused by NF consist of Schwann cells, fibroblasts, perineuronal cells, mast cells and axons embedded in an extracellular matrix. Another function of neurofibromin is to bind to microtubules that play a role in the release of adenylyl cyclase and its activity. Adenylyl cyclase plays an essential role in cognition. Neurofibromin's role in the activity of adenylyl cyclase explains why patients with NF experience cognitive impairment.