Abstract: The invention relates to methods for screening selective modulators of half transporter proteins of the ABCG family, more closely of ABCG1 and ABCG4. In particular the invention relates to methods for determining whether a substance is a selective activator, an inhibitor or a substrate of an ABCG1 or ABCG4 homodimer or of an ABCG1/ABCG4 heterodimer protein, methods for detection of ABCG1 protein in a biological sample, methods for modulating the function of said proteins, and methods for detecting the presence of and/or quantitating ABCG1/ABCG4 heterodimer activity in a biological sample. Moreover, the invention relates to isolated ABCG1/ABCG4 heterodimer proteins and antibodies selective for ABCG1 or ABCG4. The closely related human ABC half-transporters, ABCG1 and ABCG4, have been suggested to play an important role in cellular lipid/sterol regulation. ABCG1 and ABCG4 and mutants thereof have been expressed and studied by the present inventors in whole cells as well as isolated membrane preparations.