The list of human RecQ helicase comprises RecQ1, BLM (Bloom syndrome), WRN (Werner syndrome), RTS (Rothmund-Thomson syndrome), and RecQ5. Of these, the defective BLM, WRN, and RTS helicases are responsible for distinct but overlapping clinical features suggesting premature aging and an enhanced risk of cancer, which apparently stems from chromosomal instability in the cells of tissues and organs where expression of the helicase genes are specified. In an effort to obtain an animal model for these diseases, we performed gene target experiments to generate the WRN and RTS knockout mice.