Activist investor Carl Icahn is seeking to block the $67 billion merger between health insurer Cigna and pharmacy benefit giant Express Scripts, according to The Wall Street Journal.

• Icahn has not previously disclosed a stake in either company, but now apparently has taken a "sizable" position in Cigna (although it's still below 5%).

Our thought bubble: Some analysts were scratching their heads over the deal, which was viewed as a rushed attempt to copy UnitedHealth Group — and generally questionable given that Express Scripts would lose its most lucrative client in Anthem. Icahn's stake, plus the pending federal antitrust reviews, really do put this deal in jeopardy.