immune system cell that can destroy cancer cells and cells infected with viruses, fungi or certain bacteria. CTLs, also known as killer T cells, carry the CD8 marker. CTLs kill virus-infected cells, whereas antibodies generally target free-floating viruses in the blood. CTL responses are a proposed but unproven correlate of HIV immunity. (See also CD8+ T lymphocyte.) deletion: elimination of a gene either in nature or in the laboratory.