C21orf66, also known as GCFC (GC-rich sequence DNA-binding factor), is a 917 amino acid protein that localizes to the nucleus and belongs to the GCF family. Expressed ubiquitously, C21orf66 functions as a possible transcription factor and exists as four alternatively spliced isoforms, designated A, B, C and D. The gene encoding C21orf66 maps to human chromosome 21, which houses approximately 300 genes and comprises nearly 1.5% of the human genome. Chromosome 21-associated disorders include Alzheimer's Disease, amyotrophic lateral sclerosis and, most notably, Down Syndrome (also known as trisomy 21.