Fluorescence micrograph of an assay for cell fusion. Pig kidney cells were labeled with two different dyes (red, green) and mixed together to grow on a plastic dish. The cells were then infected with a strain of pseudo-rabies virus with a fluorescently labeled virus component (a capsid protein fused to cyan fluorescent protein, rendered as blue in this image). If spread of infection caused cell fusion, we would expect to see yellow-orange cells as green and red dyes mixed together.