Complex sentence: The caveat, however, is that the recombination reaction can happen backwards, rendering the cassette exchange inefficient, and that sequence excision can happen in trans instead of in cis cassette exchange event.
Split into simpler sentences: The caveat, however, is that the recombination reaction can happen backwards, rendering cassette exchange inefficient. In addition, sequence excision can happen in trans instead of a in cis cassette exchange event.