How do Transwell assays help model biological barriers in the lab? What does TEER tell us about barrier integrity and tight-junction strength? How can these assays advance our understanding of drug transport and disease? Biological barriers are critical for maintaining cellular homeostasis and protecting organs from harm caused by pathogens, drugs, and toxins. Like a country’s border crossing, a biological barrier decides who, or more so, what, gets through, and when. These specialized cellular interfaces, such as those in the brain, skin, kidney, and retina, regulate how nutrients, metabolites, and signaling molecules are exchanged between different body parts. While we often...