Check for insolubility - this is important because T7 expression often leads to very high production of protein that can result in the target protein becoming insoluble. Solutions around this are:

» Induce at lower temperatures (as low as 12 - 15°C overnight)
» Reduce IPTG concentration to 0.01 - 0.1 mM
» Induce for less time (as little as 15 minutes)
» Induce earlier in growth (OD₆₀₀ = 0.3 or 0.4)