The mucins represent a heterogeneous family of highly glycosylated proteins synthesised in epithelial cells.
12 genes (MUC1, MUC2, MUC3A, MUC3B, MUC4, MUC5AC, MUC5B, MUC6, MUC7, MUC8, MUC9 and MUC12) are known in humans whereas MUC1, MUC3A, MUC3B, MUC4 and MUC12 are in part membrane bound. The MUC genes share the common feature of having a centrally located region of sequence that encodes tandem repeats (TRs) that may comprise more than 50% of the apomucin. This region contains a large number of potential sites for O-linked glycosylation. The repeat units vary in length from 24 nucleotides in MUC5AC to 507 nucleotides in MUC6.
Most of the mucin genes exhibit a high degree of genetically determined polymorphism, due to Variation in Number of Tandem Repeats (VNTR) in the TR domain. An association between short alleles of MUC1 and gastric cancer has been reported.
The MUC7 gene is located on chromosome 4q13-q21 and encodes a low Mr secreted mucin (150-220 kDa), found in saliva, previously called MG2. It is thought to function in a protective capacity by promoting the clearance of bacteria in the oral cavity and to aid in mastication, speech and swallowing. It has been shown to interact with a variety of bacteria. MUC7 has been shown to be expressed in the bronchial tree as well as in the salivary glands.
MUC7 contains unique 5' and 3' regions (with five potential sites for N-glycosylation) and a central tandem repeat domain, which in the most common allele comprises six tandem repeats each of 69 nucleotides (23 amino acids), with many potential sites for O-glycosylation and makes sup 18% of the coding region. Theses repeats are very similar but not identical, incorporating between one and seven nucleotide substitutions between them.