Shares of the clinical-stage metabolic disorder specialist Viking Therapeutics (NASDAQ: VKTX) were down by 6.45% on moderate volume as of 1:42 p.m. ET Thursday afternoon. Small- and mid-cap biotech stocks are under pressure today following the Federal Trade Commission's (FTC) decision to sue over the proposed merger between Amgen and Horizon Therapeutics earlier this week. Before investors get too concerned, however, it's important to understand what's actually driving this takeover speculation.