This schematic diagram shows an antibiotic-sensitive target protein molecule (in red). It has an antibiotic-binding site which is close to its substrate-binding site, so that when the antibiotic (in green) is bound, the substrate (in blue) is excluded. A mutation that alters the structure of the target protein such that it no longer has an antibiotic-binding site makes the target antibiotic-resistant as it can now bind its substrate in the presence of the antibiotic.