Complex sentence: Harris's research interests were primarily focused on cancer cells and of their differences from normal cells, and later on the possibilities of genetic modification of human cell lines with material of other species in order to increase the range of genetic markers.
Split into simpler sentences: Harris's research interests were primarily focused on cancer cells and their differences from normal cells. He later studied the possibilities of genetic modification of human cell lines with material of other species in order to increase the range of genetic markers.