Complex sentence: The pacing horses studied were all homozygous for the "DMTR3" mutation, but not all horses with the homozygous mutation could pace, suggesting other factors had to come into play for that gait to occur.
Split into simpler sentences: The pacing horses studied were all homozygous for the DMTR3 mutation. But not all horses with the homozygous mutation could pace, suggesting other factors had to come into play for that gait to occur.