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. This ability does not rival it in power, however, as it was unable to regenerate missing limbs or organs. 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, with the exception of a severed arm, which Madara instead replaced with the arm of a White Zetsu clone.

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.