Tamm-Horsfall protein (THP) was first described as uromucoid by Morner in 1895. THP, which seems to be almost identical to uromodulin extracted from urine of pregnant women [1], is a 85-kD glycoprotein which is the most abundant protein in human urine under physiological conditions [for reviews see 2, 3]. THP contains 639 amino acids with eight potential glycosylation sites [4].

In its native form, THP exists in a polymeric form with a molecular weight of about 1–10! 106 daltons which can be dissociated into monomeric subunits of about 85 kD [5]. The main part of the molecule is a polypeptide (about 70% of molecular weight) which is identical in different species [5]. This glycoprotein contains a great amount of acidic amino acids which explains its low isoelectric point [6]. About 30% of the THP/uromodulin molecule consists of carbohydrates, comprising at least five N-glycosidically bound sugar chains [7]. Human THP contains large amounts of sialic (neuraminic) acid (about 5% of total molecular weight)[5]. It was isolated from urine by Tamm and Horsfall [8] by repeated precipitation with 0.58 M NaCl and biochemically characterized as glycoprotein which inhibits viral hemagglutination. This glycoprotein was rediscovered in 1985 by Muchmore and Decker [9] as uromodulin which was isolated from the urine of pregnant women using lectin