Important phases monitored by Gel electrophoresis in the retroposons enrichment strategy in silver carp (A) and bighead carp (B). 1. Step Digestion of genomic DNA. HaeIII can cleave the genomic DNA and generate size fragments evenly (< 4 Kb). 2. Step PCR enrichment. 30 cycles of step PCR enrichment detected the size range of PCR-enriched HaeIII-fragmented DNA, and finally, after being optimized, only 12 cycles of the PCR enrichment was conducted for maintaining the complexity of DNA molecules. The enriched HaeIII-fragmented pool was manly ranging from 500–2000 bp. 3. Step Adapter PCR. 30 cycles of Adapter PCR detected the size of captured target fragments cloning into vector, and finally, after being optimized, only 15 cycles was done to keep the complexity of DNA molecules.