the research group led by Jesse Roth

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May I note that changing the previous phrasing "During this NIH period De Meyts became a member of a research group led by Jesse Roth..." to: "During this NIH period De Meyts became a member of the research group led by Jesse Roth.." is justified in the context concerned because Professor Roth did not lead more than one research group. Determining this is achieved by replacing the indefinite article a with the definite article the.