Human papillomavirus particles. Coloured transmission electron micrograph (TEM) of an intracellular (inside a cell) crystalline array of particles (green and purple) of the human papillomavirus (HPV). This regular arrangement of the particles is like the regular structure of the atoms of a crystal. HPV is the cause of warts, growths that are commonly found on the hands, the soles of the feet, the body's mucous membranes and also on the genitals. HPV has also been linked with some skin and cervical cancers. Magnification: x22,000 when printed 10cm wide.