The image below left shows a cartoon representation of signalling through a specific receptor tyrosine kinase (RTK). Phosphorylation of downstream proteins is indicated by a "P" within a yellow circle. (This is the same pathway as the previous question.)

The image below right is experimental data on this pathway, obtained either in the presence or absence of several drugs, each affecting an unknown protein in the pathway. The upper panels show total protein and phosphorylation states of a number of key proteins in this signalling pathway. The lower image is of quantitative-RT-PCR for detection of expression of a known target gene at the end of this signalling pathway.

Separate experiments (not shown) concluded that none of these drugs had an effect on the activity RAS or RAF.

Which drug appears to be affecting the kinase activity of MEK but is still allowing transcription of the target gene?