Which of the following is NOT considered a mutagen?

Myofibrils are not considered mutagens because they are structural components of muscle cells responsible for contraction and do not directly cause changes to the DNA sequence. Mutagens, on the other hand, are agents that can induce changes in the genetic material of organisms, leading to mutations.

X-rays, being a form of ionizing radiation, can damage DNA and lead to mutations. Similarly, certain viruses can insert their genetic material into host DNA, potentially causing mutations. Chemicals found in tobacco smoke, such as polycyclic aromatic hydrocarbons and nitrosamines, are known to interact with DNA, leading to mutagenic effects. Thus, myofibrils do not fall into the category of mutagens, as they do not have any involvement in altering genetic information.