The purpose of this experiment is to determine the phosphorylation ratio of pT vs pY ERK present in ERK phosphorylation reaction by activated MEK E203K or MEK WildType. Aliquots were collected at 3 minutes 15 seconds, the time at which the mono-phosphorylated ERK is at its maximal. To determine the ratio between pT and pY ERK, synthetic AQUA peptides corresponding to the pY- and the pT-phosphorylated ERK were spiked into the sample. The two peptides have identical mass but they can be umabiguguously distinguished by their elution times and fragmentation spectra.