Matrix protein of human T-cell leukaemia virus, molecular model. A matrix protein binds together the outer protein coat of the virus with its inner core. The T-cell leukaemia virus causes several diseases, including adult T-cell leukaemia, a cancer of the T-cells (a type of white blood cell) of the immune system. In this model, solid colours represent the secondary structure of the protein (spiral ribbons are alpha helices), and the faded tubular structure behind represents the individual atoms and bonds.