The Sp100 protein is a constituent of nuclear domains, also known as nuclear dots (NDs). An ND-targeting region that coincides with a homodimerization domain was mapped in Sp100. Sequences similar to the Sp100 homodimerization/ND-targeting region occur in several other proteins and constitute a novel protein motif, termed HSR domain (for homogeneously-staining region). The HSR domain has also been named ASS (AIRE, Sp-100 and Sp140). This domain is usually found at the amino terminus of proteins that contain a SAND domain pfam01342.