Check for insolubility - this is important because high expression and high production of protein can result in the target protein becoming insoluble. Potential solutions for this are:
» Induce at lower temperatures (as low as 15°C overnight)
» Reduce IPTG concentration to between 0.01 mM and 0.1 mM
» Induce for less time (as little as 15 minutes)
» Induce earlier in growth (OD₆₀₀ = 0.3 or 0.4)