AbstractHuman JNK stimulatory phosphatase‐1 (JSP‐1) is a novel member of dual specificity phosphatases. A C‐terminus truncated JSP‐1 was expressed in Escherichia coli and was crystallized using the sitting‐drop vapor diffusion method. Thin‐plate crystals obtained at 278 K belong to a monoclinic space group, C2, with unit‐cell parameters a = 84.0 Å, b = 49.3 Å, c = 47.3 Å, and β = 119.5°, and diffract up to 1.5 Å resolution at 100 K. The structure of JSP‐1 has a single compact (α/β) domain, which consists of six α‐helices and five β‐strands, and shows a conserved structural scaffold in regard to both DSPs and PTPs. A cleft formed by a PTP‐loop at the active site is very shallow, and is occupied by one sulfonate compound, MES, at the bottom. In the binary complex structure of JSP‐1 with MES, the conformations of three important segments in regard to the catalytic mechanism are not similar to those in PTP1B. JSP‐1 has no loop corresponding to the Lys120‐loop of PTP1B, and tryptophan residue corresponding to the substrate‐stacking in PTP1B is substituted by alanine residue in JSP‐1. Proteins 2007. © 2006 Wiley‐Liss, Inc.