The C-int::GAL4(21-93)::GS hemidriver contains the gp41-1C split intein fragment fused to sequence encoding amino acids 21-93 of the Saccharomyces cerevisiae GAL4 gene ( SGDID:S000006169 ), a truncated ligand-binding domain from the Homo sapiens progesterone receptor (PGR, HGNC:8910) and the transcriptional activation domain from the Homo sapiens 'p65' gene (RELA, HGNC:9955). The GAL4 fragment contains only part of the DNA-binding domain and is transcriptionally inactive. C-int::GAL4(21-93)::GS forms one half of a 'split-intein GAL4' system: it is compatible with the GAL4(1-20)::N-int hemidriver that contains the N-terminal 20 amino acids of GAL4 fused to the gp41-1N split intein fragment (FBrf0256689). The gp41-1N and gp41-1C split intein fragments form a complementary pair which show efficient protein trans-splicing and a lack of cross-reactivity with other split intein fragments (PMID:22753413). In the 'split-intein GAL4' system, a functional transcriptional activator ('driver') is only reconstituted at the intersection of the expression patterns of the two hemidrivers; association between the gp41-1N and gp41-1C split intein fragments results in their self-excision and trans-splicing of the adjacent polypeptide sequence from each hemidriver, producing a single functional GAL4::GS driver protein (composed of a reconstituted complete GAL4 DNA-binding domain, the truncated PGR ligand-binding domain and the p65 transcriptional activation domain). The presence of the PGR ligand-binding domain in the reconstituted GAL4::GS driver means that transcriptional activation is expected to be dependent on the presence of the anti-progestin mifepristone (RU486) (FBrf0256689).