Philadelphia is an important center for medicine, a distinction that it has held since the colonial period. The city is home to the first hospital in the British North American colonies, Pennsylvania Hospital, and the first medical school in what is now the United States, at the University of Pennsylvania (Penn). Penn, the city's largest private employer, also runs a large teaching hospital and extensive medical system. There are also major hospitals affiliated with Temple University School of Medicine, Drexel University College of Medicine, Thomas Jefferson University (Thomas Jefferson University Hospital), and Philadelphia College of Osteopathic Medicine. Philadelphia also has three distinguished children's hospitals: Children's Hospital of Philadelphia, the nation's first pediatric hospital (located adjacent to the Hospital of the University of Pennsylvania), St. Christopher's Hospital, and the Shriners' Hospital. In the city's northern section are Albert Einstein Medical Center, and in the northeast section, Fox Chase Cancer Center. Together, health care is the largest sector of employment in the city. Several medical professional associations are headquartered in Philadelphia.