Figure 1.

Electron microscopy images of human and mouse mammary viruses taken in 1971. (a) Electron microscope image (×180,000) of particles from human milk. These particles are almost certainly the virions of the HHMMTV. (b) Image (×180,000) of particles from mouse milk. These particles are of the MMTV. The blurred edges of the particle images appear to be due to protein surface projections on the membrane of the viruses. The morphological characteristics of the particles in both the human and mouse milks appear to be almost identical and are unique among all viruses. When these characteristics are considered in conjunction with the 98% homology of the nucleotide sequences between the HHMMTV and MMTV, it appears likely that these viruses are variants of each other. Reprinted by permission from Nature [2] copyright 1971, Macmillan Magazines Ltd