8.  Defined Contribution Plan

The Company maintains a defined contribution plan covering substantially all domestic employees. Participating employees may make contributions to the plan, through payroll deductions. Matching contributions are made by the Company equal to 50% of the employee’s contribution to the extent such employee contribution did not exceed 6% of their compensation. During the years ended December 31, 2018, 2017 and 2016, the Company expensed approximately $264 thousand, $237 thousand and $211 thousand, respectively, related to this plan.