When a major trading firm stumbles, the tremors travel far — and regulators on both sides of the Atlantic are no longer willing to wait for the ground to crack. The Federal Reserve and Bank of England have begun scrutinizing how deeply banks are entangled with large proprietary trading operations, prompted by significant losses at Jane Street. The concern is an old one dressed in modern complexity: that the quiet, concentrated dependencies woven between institutions can transform a single firm's misfortune into a systemic unraveling. In tightening their gaze now, regulators are attempting the