Updated: May 22, 2020
Etc. is an abbreviation for the Latin phrase 'et cetera'. Given that it is an abbreviation, the use of etc. requires a period. If 'etc.' is used at the end of a sentence, then it only requires the period that usually accompanies it. For instance,
Protein X is expressed in a number tissues including muscle, spleen, bone, etc.
However, if 'etc.' is used in the middle of a sentence, it may require a comma after the period. For instance,
Muscle, brain, spleen, etc., express a negligible amount of protein X.
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