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. These glycoproteins play important roles in the protection of the epithelium and are implicated in epithelial renewal and differentiation. 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. Expressed in trachea, duodenum and intestine. Lower expression in stomach, salivary glands, liver, gallbladder, and kidney. 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. 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. The sequence differs significantly from the reference genome sequence for various reasons. The sequence differs significantly from the reference genome sequence for various reasons. The sequence differs significantly from the reference genome sequence for various reasons. The sequence differs significantly from the reference genome sequence for various reasons.