San Diego-based DexCom, a developer of continuous glucose monitoring systems, said Monday afternoon that Donald A. Lucas has resigned from the firm's board of directors. Lucas is Founding Managing Member at RWI Group, which was an investor in DexCom before the firm's IPO in 2005. Dexcom President and CEO Terry Gregg said in a statement that Don's committment to the firm "far exceeded that of a typical venture capital investor."