For the slide used for testing background correction, 100 nM PKA (Sigma, St. Louis, MO, USA) was spotted at 96 different places as positive control. The slide was incubated with 200 μl of kinase buffer (100 mM Tris pH 8.0, 100 mM NaCl, 10 mM MgCl 2 , 20 mM glutathione, 20% glycerol) plus 0.5 mg/ ml BSA, 0.1% Triton X-100, and 2 μl 33 P-γ-ATP in a humidified chamber at 30°C for 1 hour. The slide was then washed twice with 10 mM Tris pH 7.4, 0.5% SDS and once with double distilled H 2 O before being spun dry and exposed to X-ray film (Kodak, Rochester, NY, USA).