Complex sentence: Since cancer cells generally proliferate unrestrictively more than do healthy cells they are more sensitive to DNA damage, and alkylating agents are used clinically to treat a variety of tumours.
Split into simpler sentences: Since cancer cells generally proliferate unrestrictively more than healthy cells do, cancer cells are more sensitive to DNA damage-such as being alkylated. Alkylating agents are used clinically to treat a variety of tumours.