Online supplemental material. Fig. S1 shows the binding characteristics of control mAbs. Fig. S2 shows further binding characteristics of the neutralizing mAbs. Fig. S3 shows further analysis of pandemic H1N1-induced plasmablast somatic mutations. Fig. S4 shows experiments demonstrating the therapeutic control of pandemic H1N1 viral titers in lungs after mAb treatment. Tables S1-S3 provide detailed characteristics concerning the variable gene sequences cloned from pandemic H1N1 induced plasmablasts. Online supplemental material is available at http://www.jem.org/cgi/ content/full/jem.20101352/DC1. Annual Symposium of the Society for the Study of Inborn Errors of Metabolism  A family of eukaryotic proline racemase-like genes has recently been identified. Many members of this family, including a human protein called C14orf149 lack a cysteine residue at the 300 position which is criticial for the racemase activity. The function of these enzymes has remained unresolved until now. We demonstrate that human C14orf149 catalyzes the degradation of trans-3-hydroxy-L-proline to delta-1-pyrroline-2-carboxylate (P2C). This is the first enzyme of this subclass of proline racemase-like genes for which the enzymatic activity has been resolved. It is also the first human enzyme that acts on 3-hydroxyproline. A mutant enzyme in which the threonine-300 in the active site is mutated back into cysteine, regains 3-hydroxyproline epimerase activity. This result and the evolutionary relationship between enzymes of the proline racemase-like family, suggests that a single mutation gave rise to this subclass of enzymes.