Carl Icahn is preparing a proxy fight at Illumina ($ILMN@US), arguing the company cost its shareholders roughly $50 billion by moving forward with a risky acquisition despite opposition from regulators. The billionaire activist plans to nominate three people to the company’s board, according to a letter Mr. Icahn plans to send to Illumina’s shareholders Monday that was viewed by The Wall Street Journal. Illumina was valued at more than $70 billion in the summer of 2021 but has plummeted to about $30 billion in recent months. The biotechnology company’s takeover of cancer-screening company Grail Inc. has stalled while facing heightened competition from lower-cost rivals.