Sterol sensing domain (SSD) proteins and their phylogeny. Full-length SSD proteins can be clustered into seven major families by Clustal W: HMG-CoAr, SCP, 7-DHCR, Disp, NPC, PTC, and PTR (Thompson et al. 1997, Kuwabara et al. 2000). Sequence prefix denotes species: C. elegans (Ce), D. melanogaster (Dm), M. musculus (Mm), and H. sapiens (Hs). GenBank accession numbers are available in the Methods section.