Alpha-1B-glycoprotein (A1BG) is an approximately 80 kDa molecule that circulates in the serum. Mature human A1BG consists of five tandem Ig‑like domains and shares 44% amino acid sequence identity with mouse and rat A1BG. Alternative splicing generates an isoform that lacks the signal peptide and the first Ig‑like domain. A1BG binds with high affinity to the cysteine‑rich protein CRISP‑3. In pancreatic carcinoma, it is elevated in pancreatic juice as well as in the serum relative to normal patients. A1BG is abnormally excreted into the urine during idiopathic nephrotic syndrome (INS). Multiple breakdown fragments of A1BG are also present in INS urine, including one of 14 kDa that is found in steroid‑resistant but not steroid‑sensitive cases. A1BG is also elevated in the urine of patients with interstitial cystitis/painful bladder syndrome (IC/PBS).