Complex sentence: Production was allowed, as long as X-Men material was not used in the promotion of "Mutant X." Apparently, the title "Mutant X" itself was deemed too close to "X-Men" to be effectively leveraged.
Split into simpler sentences: Production was allowed, as long as X-Men material was not used in the promotion of "Mutant X". Apparently the title "Mutant X" itself was deemed too close to "X-Men" to be effectively leveraged.