The human genome contains 30,000 to 50,000 genes. Much is known regarding the structure, regulation, and replication of genes in animal cells. In addition, the mechanisms by which DNA is degraded have been elucidated in recent years. DNA is degraded under certain circumstances, in particular, during apoptotic cell death. Some mammalian cells, such as mature erythrocytes and cells in the lens of the eye, lose their nuclei during differentiation. In this article, the molecular mechanism of the apoptotic DNA degradation and the enucleation process of erythroid cells are discussed.