One of Sun Healthcare Group Inc.’s (NASD: SUNH) largest investors called on the board to sell the company, saying the firm is undervalued in the public markets and a sale would maximize shareholder value.

Sun, based in Irvine, Calif., operates a number of healthcare centers and provides a variety of healthcare services. Its CareerStaff division provides healthcare staffing. CareerStaff ranks No. 11 on Staffing Industry Analysts’ list of largest staffing firms with 2010 healthcare staffing revenue of $92 million

Calling for the sale is Clinton Group Inc., the investment manager for Clinton Magnolia Master Fund Ltd., which Clinton said ranks among Sun’s top 15 shareholders.

“Given the public market’s pessimism, on the one hand, and the likely enthusiasm of strategic acquirers, on the other, we believe the best way for the board of directors to maximize value for shareholders is for the company to embark on a targeted sale process, aiming to sell the company in the first half of 2012,” Clinton wrote in a letter to Sun. “We expect that Sun and its shareholders could receive a price of $6 to $8 per share in such a transaction, given the likely buyer interest (and their trading multiples) and the opportunities for efficiencies.”