(A) Kanamycin - (II) Selectable marker : Kanamycin is an antibiotic, and a resistance gene for kanamycin is often used as a selectable marker in genetic engineering. Cells that have been successfully transformed with the desired DNA will also carry the kanamycin resistance gene, and thus will survive in a culture medium containing kanamycin, while cells without the gene will not. (B) ClaI - (III) Restriction site : ClaI is a restriction enzyme used in molecular biology to cut DNA at specific sites (restriction sites). (C) Disarmed retroviruses - (I) Delivers genes into animal cells : Retroviruses are often used as vectors in gene therapy to deliver genes into animal cells. They can be "disarmed" to make them safe for use in humans or other animals. (D) Kanamycin gene - (IV) Antibiotic resistance : The kanamycin resistance (Kan^R) gene is a type of antibiotic resistance gene. It produces a protein that inactivates kanamycin, allowing bacteria containing this gene to survive in the presence of this antibiotic.