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T4-binding globulin (TBG) is the main transport protein for T4 in blood and a member of the superfamily of serine proteinase inhibitors. So far, 14 mutations leading to familial complete TBG deficiency have been reported. Eleven of these are caused by mutations leading to truncation of the molecule, and three are caused by single amino acid substitutions.We report and study the complete deficiency TBG variant found in a patient from NeuIsenburg, Germany (TBG-CDNI).Direct DNA sequencing was used to identify the TBG-CDNI mutation in the propositus, which was confirmed by allele-specific amplification. Site-directed mutagenesis and expression in Xenopus oocytes was used to study the secretion defect of TBG-CDNI and several variants by Western blot and T4-binding assay.The deletion of two nucleotides in codon 384 (1211_1212delTC) causes a frameshift altering the last 11 residues, introduces a new glycosylation site, and elongates the molecule by seven new amino acids. In contrast to normal TBG, TBG-CDNI was not secreted by Xenopus oocytes. Elongation of normal TBG by seven alanines did not affect its secretion or binding properties. On the other hand, neither disruption of its new glycosylation site nor termination of TBG-CDNI at the normal length repaired its secretion defect.In this first late termination variant of complete TBG deficiency, alteration of β-strand 5B, located in the core of the molecule, rather than elongation of the molecule or introduction of a new glycosylation site, suffices to disrupt secretion of TBG-CDNI.