When a subject particle 가 (ga) is attached, a form 누가(nuga) should be used instead of 누구가(nuguga), e.g. 누가 집에 있어요? (Nuga jib-e isseoyo?) - "Who is at home?"
Etymology
First attested in the Yongbi eocheonga (龍飛御天歌 / 용비어천가), 1447, as 누(Yale: nwu). Found in 17th-century sources as 누고 (nukwo).