Billionaire brothers Charles and David Koch have hired Stinson Morrison Hecker to represent them in a lawsuit against libertarian think tank The Cato Institute.

Stinson Morrison partners Daniel Crabtree and Heather Woodson of the Kansas City, Missouri-based firm are listed as the Kochs’ lawyers on the complaint filed in state court in Johnson County, Kansas, on Wednesday. The basis of the suit is the brothers’ claim that they should have the option to buy the shares in Washington, D.C.-based Cato that belonged to the nonprofit’s chairman, William Niskanen, before he died in October at age 78. (The Koches already own 50 percent of Cato.)