  We emphasize the complementarity of timelike and spacelike studies of deep
exclusive processes, taking as an example the case of timelike Compton
Scattering (TCS) i.e. the exclusive photoproduction of a lepton pair with large
invariant mass, vs deeply virtual Compton scattering (DVCS) i.e. the exclusive
leptoproduction of a real photon. Both amplitudes factorize with the same
generalized parton distributions (GPDs) as their soft parts and coefficient
functions which differ significantly at next to leading order in alpha_s. We
also stress that data on TCS at very high energy should be available soon
thanks to the study of ultraperipheral collisions at the LHC, opening a window
on quark and gluon GPDs at very small skewness.
