Q. Hi. I am editing a text and would like to know whether the following sentence should have a comma after the word “so”: “So let’s think about how to understand the chemical diversity of the 20 amino acids.”

A. The conjunction “so” can normally begin a sentence without the help of a comma—as can an introductory “and” or “but.” But that doesn’t rule out a comma in every case. Compare these two sentences:

“So where do you want to go next?”
“So, where do you want to go next?”

The comma after “so” in the second example puts the emphasis on that word, making it read as a sort of interjection. In fiction and related genres—particularly in dialogue—this distinction can be important. (See “ ‘Erg, no kidding?’ Interjections in Creative Writing” at CMOS Shop Talk.) In more ordinary prose, however (including your example), such a comma can usually be omitted.

Note that a comma is always omitted when the initial “so” is used as an ordinary conjunction in the sense of “with the result that” or “in order that” rather than as a purely introductory word:

So I wouldn’t miss my stop, I closed my book.