Fractions of 550 ml (20 fractions) were collected with a Piston Gradient Fractionator (BioComp, Fredericton, NB, Canada), and the density of each was determined by an Abbe's Refractometer. Viral DNA was extracted from each fraction and quantified with respect to the number of HBoV1 gc, using HBoV1-specific qPCR as described below. Those fractions containing the highest numbers of HBoV1 gc were dialyzed against PBS, and then viewed by electron microscope and used to infect HAE cultures. Observation by electron microscopy (EM)