This electron micrograph reveals both a paramyxovirus measles virus. and virions of the polyomavirus. simian virus SV40 (smaller circles). The envelope of the measles virus has broken. exposing the nucleocapsid filaments. Interest in SV40 has increased in the last several years because the virus was found in certain forms of cancer in humans. for instance mesotheliomas (rare tumors located in the lungs). brain. and bone tumors (Carbone et al.. 1994, Jasani et al.. 2001).