Jan Petersen aka Grootmeester Jan, GMJ, is a Dutch rapper, living in Leeuwarden. After performing with the rapcrews Vlieg (1997-1999) and DAC, De Amersfoortse Cooperatie (1999-now), he recorded his first soloalbum, Collaboratorium (2005). Collaboratorium contains features from D.A.C., Nuclear Family, C-Mon & Kypski and many others. He is famous for his funny wordplay and his raw beats. On stage he performs with a funkband.