precursor

noun

something that precedes and indicates the approach of something or someone
何かまたは誰かの接近に先行し、示す何か
a person who goes before or announces the coming of another
人の先を行く人、あるいは人の訪れを告げる人
a substance from which another substance is formed (especially by a metabolic reaction)
もう一つの物質はある物質から生成される（特に新陳代謝反応によって）

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