  We introduce $\mu$BERT, a mutation testing tool that uses a pre-trained
language model (CodeBERT) to generate mutants. This is done by masking a token
from the expression given as input and using CodeBERT to predict it. Thus, the
mutants are generated by replacing the masked tokens with the predicted ones.
We evaluate $\mu$BERT on 40 real faults from Defects4J and show that it can
detect 27 out of the 40 faults, while the baseline (PiTest) detects 26 of them.
We also show that $\mu$BERT can be 2 times more cost-effective than PiTest,
when the same number of mutants are analysed. Additionally, we evaluate the
impact of $\mu$BERT's mutants when used by program assertion inference
techniques, and show that they can help in producing better specifications.
Finally, we discuss about the quality and naturalness of some interesting
mutants produced by $\mu$BERT during our experimental evaluation.
