MAMSTR is a 415 amino acid nuclear protein that functions as a transcriptional coactivator by stimulating MEF-2. Containing one SAP domain, MAMSTR is expressed in spleen, placenta, skeletal muscle and brain, and exists as three alternatively spliced isoforms. The gene encoding MAMSTR maps to human chromosome 19, which consists of over 63 million bases, houses approximately 1,400 genes and is recognized for having the greatest gene density of the human chromosomes. It is the genetic home for a number of immunoglobulin (Ig) superfamily members, including the killer cell and leukocyte Ig-like receptors, a number of ICAMs, the CEACAM and PSG family and Fc receptors (FcRs).