Pyong Ko was a 21st century surgeon who discovered the genetic sequence for inhibiting the growth of cancer cells. This heralded a new age in Earth Medicine.
In 2371, Pyong Ko was one of the names the Doctor considered for naming himself. Kes sarcastically told him "that's a perfect name, Doctor." (VOY: "Fury")
This probably happened within fifty years of Zefram Cochrane's warp flight in 2063.