Final answer:
The difficulty in cloning gene X using the pUC19 vector could be due to the degradation of the foreign genomic DNA.
Step-by-step explanation:
A possible reason for the difficulty in cloning gene X using the pUC19 vector could be the degradation of the foreign genomic DNA. If the genomic DNA was left on the lab bench instead of being stored in the freezer, it may have been degraded by nucleases. This could result in unsuccessful cloning attempts as the degraded DNA may not be able to properly integrate with the plasmid vector.