The plasmid pEt-19b encoding the 37 kDa form of wild-type human protein PTP1B (residues 1 to 321) was provided by Dr. N. K. Tonks. The first chimera, designated Chimera 0, was made by substituting loop residues of wild-type PTP1B via the Q5-SDM kit (New England Biolabs), replacing with residues HVGNWPDQTAVS from the YopH WPD-loop in the corresponding region (Table ). Chimera 0 was used as the DNA template for the subsequent chimeras. Chimeras 1 to 7 were created using the polymerase chain reaction (PCR) with primers encoding residues before the mutation and the target mutation itself. The chimera DNA was then cleaved using restriction enzyme DpnI and ligated into the pEt-19b vector using T4 ligase. Each subsequent chimera was made using the previous chimera DNA as a template. The primers used are listed in Table . Chimeras studied in detail in this work are underlined in column 1. The designation "/" in the Activity column indicates an insoluble chimera for which activity could not be assessed.