Membrane-bound mucin, a family of highly glycosylated proteins that constitute the major component of the mucus, the slimy and viscous secretion covering epithelial surfaces (PubMed:10880978). These glycoproteins play important roles in the protection of the epithelium and are implicated in epithelial renewal and differentiation (PubMed:10880978). Regulates cellular behavior through both anti-adhesive effects on cell-cell and cell-extracellular matrix interactions and its ability to act as an intramembrane ligand for ERBB2. Plays an important role in proliferation and differentiation of epithelial cells by inducing specific phosphorylation of ERBB2. In polarized epithelial cells, segregates ERBB2 and other ERBB receptors and prevents ERBB2 from acting as a coreceptor. The interaction with ERBB2 leads to enhanced expression of CDKN1B. The formation of a MUC4-ERBB2-ERBB3-NRG1 complex leads to down-regulation of CDKN1B, resulting in repression of apoptosis and stimulation of proliferation. Its ability to promote tumor growth may be mainly due to repression of apoptosis as opposed to proliferation. A heterodimeric complex, composed of a mucin-4 alpha chain and a cysteine-rich transmembrane mucin-4 beta chain. Mucin-4 beta chain interacts with ERBB2 via the EGF-like domain 1. In nonpolarized cells, associates with ERBB2 and ERBB3. Isoforms lacking the Cys-rich region, EGF-like domains and transmembrane region are secreted. Secretion occurs by splicing or proteolytic processing. Forms a complex with Mucin-4 beta chain at the cell membrane. Additional isoforms exist. Expressed in the thymus, thyroid, lung, trachea, esophagus, stomach, small intestine, colon, testis, prostate, ovary, uterus, placenta, and mammary and salivary glands. Expressed in carcinomas arising from some of these epithelia, such as lung cancers, squamous cell carcinomas of the upper aerodigestive tract, mammary carcinomas, biliary tract, colon, and cervix cancers. Minimally or not expressed in the normal pancreas or chronic pancreatitis, but is highly expressed in pancreatic tumors and pancreatic tumor cell lines. Expressed early in the primitive gut before respiratory and digestive epithelial cells have acquired their tissue and cell specificity. Expressed at the basal surface of the epithelium from week 14 to 26 weeks and then predominantly localized in only parietal cells. Immediately before birth, found in the cytoplasm of the mucous columnar epithelial cells. In the embryo expressed in skin, then disappears late in gestation. Essentially composed of an array of serine- and threonine-rich tandem repeats which is highly polymorphic, the variable number of tandem repeats (VNTR) region. Proteolytically cleaved into 2 chains, mucin-4 alpha chain and mucin-4 beta chain. Highly O-glycosylated. Is predominantly N-glycosylated. The variable number of tandem repeats (VNTR) region, an array of serine- and threonine-rich tandem repeats, is encoded by a single exon (exon 2) which is highly polymorphic. Expression is a very useful predictor of poor prognosis in patients with invasive ductal carcinoma and intrahepatic cholangiocarcinoma, mass forming type (IDC,ICC-MF). Patients with IDC or ICC-MF who have high MUC4 expression had a worse survival rate than those with low MUC4 expression. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Dubious isoform produced through aberrant splice sites. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. May be preferentially expressed in tumor tissues. May be preferentially expressed in tumor tissues. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Dubious isoform produced through aberrant splice sites. Dubious isoform produced through aberrant splice sites. May be derived from an intron translation.