Regeneration Ability

The Regeneration Ability is a power lying inside the cells of Hashirama Senju, which is able to heal the injuries of anyone imbued with them in mere moments.

Acquisition
Since the healing power is an inherent component of Hashirama's cells, someone who obtains the cells also obtains their healing factor. This can be done by simply implementing the cells into one's own body, or by taking over the body of a White Zetsu. Because of their origin, their DNA is very similar to Hashirama's, as revealed by Shizune and Sakura Haruno's autopsy results.

Usage
This ability has the advantage of healing most damage that the user sustains without the need for hand seals. In this regard, Madara Uchiha compares it to Tsunade's Ninja Art Creation Rebirth — Strength of a Hundred Technique. The enhanced regeneration granted by the cells is so rapid that even dozens of puncture wounds, on top of various injuries inflicted by the combined physical assault of all nine tailed beasts were completely healed in mere moments. This ability does not appear to extend to severed limbs as Madara instead replaced his severed arm with that of a White Zetsu clone's.

Improvement
After becoming the Ten-Tails' jinchūriki, Madara claimed that it was impossible for him to be defeated as he gained a form of "immortality" from how far his regeneration's ability had ascended, as he regenerated the half of his torso blown away by Night Guy and regenerated even after Sasuke Uchiha fully bisected him at the waist.

Trivia

 * Despite the Regeneration Ability being similar to a kekkei genkai, it was never said to be one.
 * With the genetic tissue transplanted into Madara and the artificial limbs for Obito each comprised of Hashirama's DNA, they both not only gained Hashirama's healing abilities, but also they no longer required any food or water to survive. However, it is unknown if these two traits are related or if Hashirama himself was also endowed with the latter.
 * Though Jūgo was in possession of Hashirama's cells at one point, he automatically converted them into his own before having a chance to use this ability.
 * Those who possess a fragment of the Stone of Gelel inside their bodies will gain regenerative powers comparable to this ability.
 * In the anime, Mugino states to be able to use Wood Release a person must have the First Hokage's cells, which suggests that the Wood Release user Moegi Kazamatsuri has access to this ability.
 * In the anime, rumors have spread that the Regeneration Ability also will restore one's youth and extend their longevity. Konoha-nin however have denied such claims.