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Cainan (from Hebrew: קֵינָן Kênān) is mentioned in the Septuagint, the Greek translation of the Book of Genesis, the Book of Jubilees and the genealogy of Jesus given in Luke 3:36 in the New Testament. He is described as a son of Arpachshad and father of Salah, who lived in the time between Noah and Abraham. A patriarch of a similar name is mentioned in all versions of Genesis as living before the flood.
The postdiluvian Cainan doesn't appear in the Hebrew Masoretic text of Genesis, where Arpachshad is noted as the father of Salah. He is also omitted from the writings of the Jewish historian Josephus. Early Christian writers Irenaeus and Eusebius of Caesarea believed his presence to be an error mainly on the basis of his omission from the Masoretic (Hebrew) version and many subsequent interpreters followed this. (This is impossible since Irenaeus died circa 202 and Eusebius died in 339. The Masoretes were scribes that worked between the 6th and 10th Century. .) However Helen Jacobus has argued that the omission from the Masoretic text is deliberate. 
Despite his name being omitted from the Hebrew text, a substantial number of traditions about this other Cainan exist in the history of literature:
According to the Book of Jubilees, Cainan was taught to read by his father, and he found, carved on the rocks by former generations, an inscription preserving the science of astrology as taught by the Watchers, who had rebelled from God before the deluge. He is also stated to have married a daughter of Madai named Melka.
The Sefer ha-Yashar describes Cainan, the possessor of great astrological wisdom, which had been inscribed on tables of stone, as the son of Seth and not of Arpachshad; i.e., the antediluvian Kenan grandson of Seth according to the Bible.
In The Patriarchal Age: or, the History and Religion of Mankind (1854), George Smith writes: