STOCKHOLM (AP) — Americans Robert Lefkowitz of Duke University and Brian Kobilka from Stanford have won the 2012 Nobel Prize in chemistry.

The Royal Swedish Academy of Sciences said Wednesday the two researchers had made groundbreaking discoveries about proteins that let body cells respond to signals from the outside.

Lefkowitz is an investigator at the Howard Hughes Medical Institute and professor at Duke University Medical Center in Durham, North Carolina.

Kobilka is a professor at Stanford University School of Medicine in California.

The Nobel week started Monday with the medicine prize going to stem cell pioneers John Gurdon of Britain and Japan's Shinya Yamanaka. Frenchman Serge Haroche and American David Wineland won the physics prize Tuesday for work on quantum particles.