Because not all particles which look like retrovirus are infectious, that is are viruses, one must then prove that the particles which band at 1.16g/ml are indeed infectious.

5. When the particles are introduced into secondary cultures:

1. the particles are taken up by the cells
2. the entire RNA is reverse transcribed into cDNA
3. the entire cDNA is inserted into cellular DNA
4. the DNA is transcribed into RNA which is translated into proteins

6. The cells in the secondary cultures release particles into the culture medium.

7. The particles in the secondary cultures have exactly the same characteristics as the original particles, that is, they must have identical morphology, band at 1.16 gm/ml and contain the same RNA and proteins.