Hydroxylamine is a chemical mutagen that causes, specifically, GC-to-AT transitions in phage T4, while 5-bromouracil readily causes both types of transitions. UV light causes an increase in the frequency of rII mutations in T4. Most, 57 of 62 analyzed, of these UV-induced mutations revert at a high frequency when exposed to 5-bromouracil but not when exposed to hydroxylamine. The remaining 5 mutations reverted with both mutagens. What type of mutation does UV mainly create when producing rII mutants?