After Left published a paper Friday predicting that President Donald Trump will crack down on the prices of aerospace industry goods, shares of $13 billion TransDigm plunged as much as 13%. That made the stock the worst performing on the S&P 500 around noon that day. It also caused the company to shed $1.4 billion in market cap by the market’s close.

“President Trump has become to aerospace what Hillary was to pharmaceuticals,” Left wrote. “He has already made lowering prices for military aircraft a pillar of his transition into office.”

Left pointed to two of Trump’s tweets as a sign that the president won’t be soft on aerospace and defense. On Dec. 6, Trump wrote that the cost of a Boeing Air Force One is “out of control.” Roughly a week later, he called out the cost of Lockheed and Martin’s F-35, writing that “billions of dollars can and will be saved on military (and other) purchases after January 20.”

The short seller argues that Trump should target TransDigm in the future because the company appears to have arbitrarily hiked the price of its goods.

TransDigm is also highly susceptible to Trump’s influence because the Department of Defense is TransDigm’s single largest customer.

Later on Friday, Credit Suisse analyst Robert Spingarn defended the company. While he conceded that TransDigm’s goods are expensive, he added that the company has a higher growth rate and profitability than its peers. Additionally, if Trump tried to lower the prices, investors would see the warning signs.

“If the government were to pursue lower prices from TransDigm, we would expect this to be a lengthy process,” Spingarn wrote. “One thing we can’t account for is ‘tweet risk,’ but we think the likelihood of this making it to Trump’s tweet desk is low, and the likelihood of singling out a single aerospace parts supplier is even lower.”

On the same day, the S&P 500 rose 0.34%.