We present a mutation testing framework for the Erlang functional programming language. Mutation testing evaluates a test set by mutating the original System Under Test (SUT) and measuring the test set’s ability to detect the change. Designing mutation operators can be difficult, since they must modify the original program in a way that is both semantically significant, and a realistic simulation of a potential fault (either a fault with the system in its real context, or a common programmer error). The principal contribution of this work is the mu2 framework, which leverages the Wrangler refactoring API to allow users to specify their own mutation operators. The framework makes it possible to quickly and clearly define mutation operators that can have complex and subtle effects on the SUT. This allows users to define domain-specific operators that can simulate faults that are of particular relevance to their project, rather than relying on standard operators. The mutation testing framework was evaluated in an industrial setting and compared to code coverage test adequacy metrics. It was found to be a valuable compliment to code coverage techniques, since it was able to uncover additional testing limitations that could not be easily identified by coverage alone.