RO5126766 will be administered initially as a single daily dose of 4mg using either a twice weekly dosing schedule (Monday-Thursday) or a 3 times per week dosing schedule (Monday-Wednesday-Friday). The daily dose will be given orally according to the dosing schedule. A cycle of treatment will comprise 28 days. In Part I of the study both the twice weekly and the three times per week schedules will be assessed. In the absence of dose limiting toxicities 6 patients will be recruited to each dosing schedule cohort. In the event of a DLT being observed in Part I of the study, a single dose reduction to 3.2 mg will then be instigated and up to 6 patients will be recruited to that dosing schedule at the lower dose. The optimal dosing schedule determined in Part I will be taken forward to Part II and 20 patients with tumours harbouring BRAF, KRAS and NRAS mutations will be enrolled.