STOCKHOLM, Sweden – Americans Robert Lefkowitz and Brian Kobilka have won the 2012 Nobel Prize in chemistry.

The Royal Swedish Academy of Sciences cited the two researchers Wednesday “for studies of G-protein-coupled receptors.”

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.

The Nobel Prizes were established in the 1895 will of Swedish industrialist Alfred Nobel, the inventor of dynamite. Each award is worth 8 million kronor, or about $1.2 million.