The invention provides a protein having an amino acid sequence as in SEQ ID 7, 8, 11 or 12, similar proteins, naturally occurring variants and homologous functional equivalents thereof, as well as the use of a polynucleotide encoding such protein, for use in an assay and for use in a method of modulation of transcriptional activity promoted by a responsive nuclear receptor and a coactivator, which coactivator is the mentioned protein and is named COASTER, and which nuclear receptor is preferrably a steroid receptor.