Okuma, Honda, Ichikawa et al.: "In vitro SUMO-1 modification requires two enzymatic steps, E1 and E2." in: Biochemical and biophysical research communications, Vol. 254, Issue 3, pp. 693-8, 1999 (PubMed).
Desterro, Rodriguez, Kemp et al.: "Identification of the enzyme required for activation of the small ubiquitin-like protein SUMO-1." in: The Journal of biological chemistry, Vol. 274, Issue 15, pp. 10618-24, 1999 (PubMed).
Gong, Li, Millas et al.: "Molecular cloning and characterization of human AOS1 and UBA2, components of the sentrin-activating enzyme complex." in: FEBS letters, Vol. 448, Issue 1, pp. 185-9, 1999 (PubMed).