TM4-B, also known as TSPAN16 (tetraspanin-16) or TM4SF16 (transmembrane 4 superfamily member 16), is a 245 amino acid multi-pass membrane protein that belongs to the tetraspanin (TM4SF) family. Members of the TM4SF family mediate signal transduction events that play a role in the regulation of cell development, activation, growth and motility. TM4-B is broadly expressed in most human tissues and cell lines including neural and bone marrow derived tissues. The gene that encodes TM4-B contains nearly 31,000 bases and maps to human chromosome 19p13.2. Chromosome 19 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 families and Fc receptors (FcRs).