Cloning an Oncogene. In the first cycle, human tumor DNA is put into "normal" mouse fibroblasts. In the second cycle, DNA extracted from "transformed" mouse fibroblasts is put into a new batch of "normal" mouse fibroblasts. Genomic DNA is cloned from the second batch of "transformed" mouse fibroblasts. The genomic library is screened with an Alu probe to identify clones of "human" DNA containing an oncogene. Human DNA is shown in red, mouse DNA is shown in black, the oncogene is shown as the blue segment, and Alu sequences are shown as green dots. Alu sequences are repeated randomly over a million times throughout the human genome, but are not present in the mouse genome.