The Latin loan phrase sui generis, which translates literally to of its own kind, is used in English to mean unique. The phrase is singular and doesn’t conventionally apply to plural nouns (as only one thing can be unique in a certain way). Yet writers often disregard the phrase’s literal Latin meaning and apply it to plural nouns anyway.
Sui generis is long established in English, so there’s no need to italicize it in ordinary use.
But Webb is sui generis, and I doubt anyone agrees with him on everything. [Washington Post]
What is taking place in Egypt today is the result of sui generis social, political, cultural and even geographic phenomena. [Center for a New American Security]
Thompson is an extraordinary guitar player, probably the best this country has produced, an utterly sui generis talent. [Independent]