Shares of Indianapolis-based Allison Transmission Holdings (NYSE:ALSN) are up a mere 11% over the past 12 months, less than half the gains elsewhere on the S&P 500. Regardless, insiders are starting to cash out.

On Monday, majority Allison shareholders The Carlyle Group (NASDAQ:CG) and Onex Corporation announced plans to sell a total of 15 million shares of Allison common stock. The sale is expected to close on or about November 15.

According to S&P Capital IQ, private equity firms Onex and Carlyle are Allison's two largest shareholders. Each holds a 34.79% stake in the company -- 64.4 million shares apiece. Each investor will be selling shares pro rata to its current holdings in the company, using Credit Suisse as their underwriter to further sell the shares to other buyers.

Today's announced sale represents the liquidation of about 12% of both Onex's and Carlyle's stakes in the company. Following the sale, the shareholders note that they will retain a total 61.4% interest in Allison -- approximately 111.9 million shares in total.

Allison shares, which traded down 0.4% in regular trading Monday, dropped a further 0.7% in after-hours trading on this news.