PXR is touted as a master xenobiotic receptor that is activated by a wide variety of structurally diverse compounds, so its activation in the FACTORIAL bioassays was not surprising. Its measured activity further supports the idea that the toxicity to zebrafish embryos was caused by shed nanoplastics and oligomers. While PXR activity can be challenging to interpret, owing to the receptor's ligand binding promiscuity, predictive models of PXR ligands have found that ester groups can be potent activators of human PXR. Because PCL is a polyester, this suggests that PCL breakdown products (i.e., nanoplastics and oligomers) could be the source of bioactivity in the in vitro bioassays. The tentative identification of PCL oligomers in the methanolic extracts supports this idea. Because ligands for human and zebrafish PXR are not wholly identical, the activation of zebrafish PXR by PCL and its degradation products will require further investigation.