The process of geranylgeranylation can be catalyzed by either geranylgeranyl transferase I (GGTase I) or Rab GGTase (also GGTase II).
In the case of the K- and N-Ras forms of Ras, when cells are treated with FTIs, these forms of Ras can undergo alternate prenylation in the form of geranylgeranylation.