Abstract: Disclosed are N-(phenyl)-1H-indole-3-sulfonanmide and N-(phenyl)-1H-pyrrolo[2,3-b]pyridine-3-sulfonamide derivatives of formula I substituted at the 1- and 6-positions of the 1H-indole or 1H-pyrrolo[2,3-b]pyridine moiety, as well as structurally related compounds (e.g. the N-pyridin-2-yl and the N-pyridin-3-yl analogues thereof). The present compounds have GPR17 modulating properties which are useful for treating or preventing a variety of CNS and other diseases, in particular for preventing and treating myelinating disorders such as multiple sclerosis.