Kallikrein-related peptidase 4 (KLK4) in Mmp20 null mice. Kallikrein-related peptidase 4 elutes from a reverse-phase high-performance liquid chromatography (RP-HPLC) column between 50 and 52.5 min. Left: enamel proteins (1.2 mg) from wild-type, Mmp20 null, and Klk4 null mice were fractionated by RP-HPLC and three fractions were collected, from (i) 45–50 min, (ii) 50.0–52.5 min, and (iii) 52.5–55.0 min. Right: zymography and western blot analyses of the three fractions. Active KLK4 was detected in the Mmp20 null mice migrating as a doublet. An unknown proteolytic activity migrating at 40 kDa was observed only in the Mmp20 null mice. Triangles indicate KLK4 bands; the arrowhead marks an unidentified 40-kDa protease.