I remember in the later stages of the anime, there was a flashback episode about someone who also researched genetic cells, seemingly named something like Hiruko.
Uchiha Gen had some vague memories of this part of the plot, only remembering that this scientist seemed to have wholly and indiscriminately fused various Kekkei Genkai of the Shinobi World—a technique called Chimera.
Not to mention anything else inside the human body, the viruses, bacteria, and genetic diseases alone are countless. This guy even fused summon animals, no wonder he was just a bit-part extra.
Uchiha Gen's approach and direction for becoming stronger were somewhat similar to Hiruko's, but he was much more rigorous. He only took the most core genes; although growing stronger this way required a very long time to slowly mature, at least no mutations would occur.
Wait another half year, until August or September, before fusing Izuna's portion. As for Hashirama's cells... coincidentally, there won't be any issues for the next ten-odd years anyway.
A week was not long. Living in the dire straits of the Warring States Period, one lived one day at a time, never knowing when a passing rogue shinobi or samurai would take their life. They could only sharpen their heads and drill into towns, because at least there was the protection of domain guards there.
The force established by Uchiha Gen's pawns also employed quite a few samurai, but secretly they were also cultivating their own shinobi. However, because the talented clansmen were all sent to the Clan Land, the quality ended up a bit uneven, with the highest strength being merely Chunin.
A new day began, and it was also the promised one week later. Uchiha Gen uncharacteristically put on a suit of armor. Come to think of it, Warring States armor did