  Pre-trained language models (PLMs) were considered to be able to store
relational knowledge present in the training data. However, some relational
knowledge seems to be discarded unsafely in PLMs due to \textbf{report bias}:
low-frequency relational knowledge might be underexpressed compared to
high-frequency one in PLMs. This gives us a hint that relational knowledge
might not be redundant to the stored knowledge of PLMs, but rather be
complementary. To additionally inject relational knowledge into PLMs, we
propose a simple-yet-effective approach to inject relational knowledge into
PLMs, which is inspired by three observations (namely, polymorphism, synonymous
substitution, and association). In particular, we switch entities in the
training corpus to related entities (either hypernyms/hyponyms/synonyms, or
arbitrarily-related concepts). Experimental results show that the proposed
approach could not only better capture relational knowledge, but also improve
the performance in various biomedical downstream tasks. Our model is available
in \url{https://github.com/StevenZHB/BioPLM_InjectingKnowledge}.
