Lipocalin alpha 1-microglobulin (alpha 1M) is a conserved glycoprotein present in plasma and in the interstitial fluids of all tissues. alpha 1M is linked to a heterogeneous yellow-brown chromophore of unknown structure, and interacts with several target proteins, including alpha 1-inhibitor-3, fibronectin, prothrombin and albumin. To date, there is little knowledge about the interaction sites between alpha 1M and its partners. Here, we report the crystal structure of the human alpha 1M. Due to the crystallization occurring in a low ionic strength solution, the unidentified chromophore with heavy electron density is observed at a hydrophobic inner tube of alpha 1M. In addition, two conserved surface regions of alpha 1M are proposed as putative protein-protein interface sites. Further study is needed to unravel the detailed information about the interaction between alpha 1M and its partners. (C) 2013 Elsevier Inc. All rights reserved.