13.Controlled Equity Offering

In August 2015, we entered into a Controlled Equity OfferingSM Sales Agreement (Sales Agreement) with Cantor Fitzgerald & Co. (Cantor), as sales agent, pursuant to which we may sell, through Cantor, up to an aggregate of $30.0 million in shares of our common stock. All sales of our common stock will be made pursuant to a shelf registration statement that was declared effective by the Securities and Exchange Commission (SEC) on July 13, 2015.  Cantor is acting as our sole sales agent for any sales made under the Sales Agreement for a low single-digit commission on gross proceeds.  The common stock is being sold at prevailing market prices at the time of the sale.  Unless otherwise terminated earlier, the Sales Agreement continues until all shares available under the agreement have been sold.  During the three months ended March 31, 2016, no shares of common stock were sold under the Sales Agreement. As of March 31, 2016, 1,722,312 shares of our common stock were issued under the Sales Agreement with aggregate net proceeds of $5.5 million.  As of March 31, 2016, we had approximately $24.3 million in shares of our common stock registered for sale under the Sales Agreement.