Kekkei Genkai

A kekkei genkai (血継限界, literally meaning: Bloodline Limit) is an anomaly of the DNA that allows the wielder to use unique techniques. A kekkei genkai's name describes both the anomaly and the resulting technique. Most kekkei genkai are passed down between generations of a clan, an exception being Hashirama Senju's Wood Release. Likewise, most individuals only inherit one kekkei genkai, but Mei Terumī is known to naturally possess two.
Generally speaking, there are three types of kekkei genkai:
Dōjutsu are unique eyes with special perceptive abilities, such as X-ray vision or acute attention to detail. Some dōjutsu, such as the Byakugan, are present from birth, while others, such as the Sharingan, won't awaken until specific conditions are met. Dōjutsu that are consistently inherited within a particular family, such as the Byakugan's Hyūga clan and the Sharingan's Uchiha clan, often become synonymous with the clan.
Advanced nature transformations combine two basic natures in order to create a new one, such as water and wind to make ice or fire and earth to make lava. Although it is common to use more than one nature, actually combining them requires a kekkei genkai. While some natures are commonly associated with a particular clan, such as ice with the Yuki clan, others occur within individuals with no known relationship, such as lava being used by natives of Iwagakure, Kirigakure, and Kumogakure.
Certain kekkei genkai grant their users unrivalled ways of using their own bodies. Twin brothers Sakon and Ukon can merge their own or the bodies of unrelated people. Kimimaro's Shikotsumyaku enables him to freely manipulate his bones, such as growing an additional spine for him to wield like a whip. Jūgo's body passively absorbs natural energy, allowing him to use senjutsu without requiring any training.
Kekkei genkai make their users far more powerful than the average shinobi. In times of war they are deadly on the battlefield, and are often deployed to turn the tide of the battle. They are highly effective in this regard, but this tends to create issues for them after the conflict is over: survivors of the battle that witnessed or that lost loved ones to kekkei genkai users come to fear and hate them for their actions. They are persecuted because of their abilities, forcing them to the lower rungs of society or driving them into hiding. Even the villages that employ them may stigmatise them, relying on them in combat situations but being unconcerned with their actual survival.
Even though individuals with kekkei genkai are mistreated, the actual kekkei genkai remain coveted by many. Kumogakure, for example, has made different attempts to steal the Byakugan. The Hyūga clan has taken steps to preserve the secret of the Byakugan, first by obligating members of its branch families to sacrifice their lives for members of its main family, and furthermore by branding branch members in order to destroy their Byakugan when they die. Despite these safeguards, Ao of Kirigakure was able obtain a Byakugan, and he goes to great lengths to ensure it cannot be taken back.
When attempting to transfer kekkei genkai to others, dōjutsu are the easiest to work with; the recipient merely needs to be implanted with it. If the recipient does not have a genetic predisposition for the dōjutsu, using it will demand far more chakra than is required for natural users. For nature transformation kekkei genkai, only Hashirama Senju's Wood Release has been shown trying to be recreated in others: Orochimaru attempted to reproduce Wood Release by modifying tests subjects' DNA, succeeding only with Yamato; Kabuto Yakushi perfected grafts of Hashirama that could be implanted in others, granting them Wood Release; Madara Uchiha developed a substance that, when used for artificial limbs, allows limited Wood Release usage. For bodily kekkei genkai, transferring or reproducing them appears to be prohibitively difficult: to acquire Kimimaro's Shikotsumyaku, Orochimaru planned to simply take over Kimimaro's body, a plan he had to abandon when Kimimaro's body was discovered to be dying; Kabuto succeeded to a certain extent, by creating an external extension body he can use to access the ability.
If an individual cannot gain access to a kekkei genkai directly, they can try instead to use it through the kekkei genkai's original user. Sasori could access the Third Kazekage's Magnet Release via a human puppet made from the Kazekage's body. Orochimaru and Kabuto use the Impure World Reincarnation to revive the dead – complete with any kekkei genkai they possessed during life – which are fully under their control. Kabuto also develops a method of creating mindless copies of kekkei genkai users which he generates as needed.