C2C12 myoblasts were infected with a retrovirus expressing Pax7d or with an empty virus (puro) as a control. All of the samples originated from the same common pool of parental C2C12. This pool was split into six streams. A single prep of Pax7d-puro virus was split into three volumes and used to infect three of the streams. A single prep of puro-alone virus was similarly split in three and used to infect the remaining three streams. From the point of the infection forward each stream was maintained distinct from the others. Cells were infected and grown simultaneously under identical conditions.