Major glycoprotein component of a variety of mucus gels. Thought to provide a protective, lubricating barrier against particles and infectious agents at mucosal surfaces (By similarity). Fetal and adult small intestine and fetal and adult colon. Highly O-glycosylated and probably also N-glycosylated. This gene is not currently present in the reference genome assembly (GRCh38/hg38) and is probably the result of a gene duplication of MUC3A.