Polyomavirus (PyV) is a DNA tumor virus known to cause debilitating and fatal human diseases, ranging from neurological disorders and kidney diseases to cancer. A crucial step in its infection pathway is penetration of the viral particle across the membrane of a sub-cellular compartment known as the endoplasmic reticulum (ER). However, the molecular mechanism by which PyV breaches the ER membrane remains largely mysterious. In this talk, we will clarify this enigmatic process.