A subfamily of ATP binding cassette transporters that function primarily in the transport of lipids and STEROLS across the CELL MEMBRANE. They also export UREA and various drugs resulting in MULTIDRUG RESISTANCE. They are smaller than most other ATP binding cassette proteins, consisting of six transmembrane alpha helices and a distinct N-terminal cytoplasmic ATP-binding domain, and function as homo- or heterodimers with other ABCG transporters.