Complex sentence: One of the first JAK to be targeted in drug development for medical use was JAK3 since immuno depression was seen in patients with JAK3 defects and as JAK3 is greatly restricted to the immune system it was thought it could be a new selective immunosuppressant.
Split into simpler sentences: One of the first JAK to be targeted in drug development for medical use was JAK3. It began when immuno depression was seen in patients with JAK3 defects and as JAK3 is greatly restricted to the immune system it was thought it could be a new selective immunosuppressant.