MUC5B, mapped clustered with MUC6, MUC2, and MUC5AC to chromosome 11p15.5, is a human mucin gene of which the genomic organization is being elucidated. We have recently published the sequence and the peptide organization of its huge central exon, 10,713 base pairs (bp) in length. We present here the genomic organization of its 3' region, which encompasses 10,690 bp. The genomic sequence has been completely determined. The 3' region of MUC5B is composed of 18 exons ranging in size from 32 to 781 bp, contrasting thus with the very large central exon. The sizes of the 18 introns range from 114 to 1118 bp. Some repetitive sequences were identified in four introns. The peptide deduced from the sequence of the 18 exons consists of an 808-amino acid peptide. This carboxyl-terminal region exhibits extensive sequence similarity to MUC2, MUC5AC, and von Willebrand factor, particularly the number and the positions of the cysteine residues, suggesting that this domain may be derived from a common ancestral gene. The presence in these components of a cystine knot also found in growth factors such as transforming growth factor-beta is of particular interest. Moreover, one part of this peptide is identical to the 196-amino acid sequence deduced from the cDNA clone pSM2-1, which codes for a part of the high molecular weight mucin MG1 isolated from human sublingual gland. Considering the expression pattern of MUC5B and the origin of MG1, we can thus conclude that MUC5B encodes MG1.