The human GM-CSF gene and the mouse GM-CSF gene vary by more than 60% in their DNA sequence. However, when scientists place the human gene into mice, it functions properly. What best explains why this happens?

Significant sections of genes can be filled with non-sense DNA that codes for nothing (however, functionality of nonsense DNA is being debated). Therefore, even if much of the gene doesn't match another species gene, as long as the critical areas that code for actual characteristics of the protein are present, the gene should function just fine.