Complex sentence: FGF15 is the mouse ortholog of human FGF19 (there is no human FGF15) and, where their functions are shared, they are often described as FGF15/19.
Split into simpler sentences: FGF15 is the mouse ortholog of human FGF19 (there is no human FGF15). Where their functions are shared, they are often described as FGF15/19.