Sage Transformation (仙人化, Senninka) is a way of attaining Sage Mode, using a bodily reaction exhibited by members of Jūgo's clan. By exposing their special body fluids to natural energy, they gain increased physical capabilities and the ability to perform various shape-shifting feats.
This transformation is a multifaceted ability that permits them to achieve feats ranging from creating various weapon-like appendages to even consuming other human beings, as well as a general increase in their physical capabilities, such as in strength, speed, stamina, reflexes, perception and durability, along with improved power of techniques. Once transformed, common features include eyes with yellow irides and black sclerae, altered skin colour and longer hair (hair colour change may also occur in some users).
In the experiments created by Orochimaru, this ability is referred to as "Cursed Seal Transformation" (呪印化, Juinka). A user of the Sage Transformation can transplant his or her flesh with anyone compatible with such experiments, in the events of a heavy injury, though this will cause the user's body to shrink in size and age.
Kabuto uses this transformation to enter Sage Mode.
Initially Mitsuki could only access this ability to enter Sage Mode subconsciously. Later in the anime, it was shown that he could do so freely.
Jūgo's Sage Mode.
Kabuto's Sage Mode.
Mitsuki's Sage Mode.
The term "Sage Transformation" (仙人化, Senninka) can also refer to the transformation into the jinchūriki of the Ten-Tails.
While Orochimaru discovered the Ryūchi Cave and gained the ability to mould Senjutsu chakra, he was not able to use Sage Transformation to enter Sage Mode, since he lacked a compatible body.