Mutagen

Mutagens affect DNA in different ways. Some mutagens, such as nitrogen mustard, bind to a base and cause it to make a different amino acid. These mutagens cause point mutations, because they change the genetic code at one point, so changing a protein's amino acid sequence.

Mutagens such as acridine orange work by deleting or inserting one or more bases into the DNA molecule, shifting the frame of the triplet code for an amino acid. Deletion and insertion mutations causing "frame-shift" mutations can change a long string of amino acids, which can severely alter the structure and function of a protein product.

Normal cells recognize cues from their environment and respond with specific reactions, but cells impaired by a mutation do not behave or appear normal, and are said to be transformed.