The QS entry in FlyBase represents the Neurospora crassa QA-1S repressor (UniProtKB:P11637), a negative regulator of the Neurospora crassa QA-1F transcriptional activator. QA-1S binds to the activation domain of QA-1F, such that although it can still bind the QUAS sequence it can no longer activate transcription (reviewed in PMID:1825499, FBrf0233764, FBrf0214230). QS can thus be used to further refine the expression pattern of a gene of interest expressed using the QUAS-QF binary expression system, where a transgene or modified endogenous locus encoding a 'QF' driver (based on QA-1F) is combined with a second transgene or modified endogenous locus in which the target gene of interest is downstream of QUAS sequences (reviewed in FBrf0233764, FBrf0216478, FBrf0214230). QS can potentially be used to suppress any binary expression driver or split system hemidriver that contains the interacting portion (the transcriptional activation domain) of the QA-1F gene. The suppression of a QF driver by QS can itself be suppressed by feeding flies with quinic acid, providing a further level of regulation (FBrf0210697, reviewed in FBrf0214230, FBrf0233764).