All living cells are produced during the proliferation of maternal cells. As a result, almost all living cells contain a nucleus with DNA (eukaryotes) or circular DNA (prokaryotes) as a source of genetic information. In particular, almost all human cells contain a nucleus and multiply at a certain stage of development. However, erythrocytes, platelets, as well as most cornified cells in the skin, hair, and nails, do not contain a nucleus and lack DNA. The phenomenon is associated with the loss of the nucleus in these cells during their maturation. As erythrocyte and hair cells are filled with specific proteins required for their functioning, they do not preserve the nucleus. In particular, erythrocyte is filled with hemoglobin and hair cells contain protein ceratin. A similar phenomenon is observed in plant cells where some cells (i.e., sieve tube elements) lost nucleus and DNA material. As a result, the cells without nucleus and DNA are not identified genetically.