synchronized List<Utr5prime> get5primeUtrs() {
        if (utrs == null) {
            Transcript tr = (Transcript) findParent(Transcript.class);

            // Get UTRs and sort them
            utrs = tr.get5primeUtrs();
            if (isStrandPlus()) Collections.sort(utrs, new IntervalComparatorByStart()); // Sort by start position
            else Collections.sort(utrs, new IntervalComparatorByEnd(true)); // Sort by end position (reversed)
        }

        return utrs;
    }