PIK3CA and neoplasm: The studies described here have shown that the combination of the PI3K inhibitor GDC-0941 with the MEK inhibitor PD 0325901 is more efficacious in vivo than either single agent given at the same doses, and importantly that the increased efficacy of the combination treatment correlates with a subsequent decrease in tumour [18F]-FLT uptake measured by PET after just 2 days of treatment.