Ringword puzzle #25 - clues

1: Third-party video game enhancement, abbr.
3: Let fall
5: Gene cluster with one promoter, such as lac or trp
8: Reagan or McDonald
12: Toxic amount for the average animal, abbr.
15: Freshly-discovered security flaw, abbr.
17: Full name behind Atlas Shrugged
20: From a dice, coin, or radioactivity