Kayin (קַיִן) is the firstborn of Adam and Chava. Chava names him "Acquired" (kaniti), declaring: "I have acquired a man with God" — a cry of wonder at the first human birth. He becomes a worker of the ground while his brother Hevel tends flocks.
When God accepts Hevel's offering but not his, Kayin's face falls. God confronts him directly: "If you do well, will you not be lifted up? And if you do not do well, sin crouches at the door — its desire is for you, but you must rule over it." Kayin does not heed the warning. He leads Hevel into the field and kills him. To God's question of Hevel's whereabouts he answers with words that have echoed ever since: "I do not know. Am I my brother's keeper?"
God curses the ground before Kayin and marks him — a sign of divine protection so that no one who finds him will kill him. Kayin goes east to the land of Nod, where he builds the first city and names it after his son Chanoch. His line ends with Lemech, whose sons Yaval and Yuval became ancestors of herders and musicians. The Kayin line disappears entirely after the flood; only Shet's line survives through Noach.
