Hyphens, En Dashes, Em Dashes

Q. Is the prefix “non” always used without the hyphen now? For example: nonresponse, as opposed to non-response.

A. CMOS style is to close up prefixes unless there is a reason not to. Some editors leave the hyphen in to avoid doubling a letter: non-native, pre-exist, co-owner (the second of which is listed as one word in Merriam-Webster; “nonnative” is also listed, as an equal variant for the hyphenated form). We use an en dash when the prefix goes with an open compound noun: non–United States citizen. And we use a hyphen when the prefix goes with a hyphenated compound: non-English-speaking.