I am currently doing some western blotting analysis to identify some specifically regulated phospho-proteins. I am using 4G10 platinum antibody (millipore) to recognize tyrosine-phosphorylations. When studying protein phosphorylation I usually blot the membrane in 5% BSA rather then in 5% milk because it is known that milk contains casein (that may be phosphorylated thus increasing the background).

The protocol of my antibody suggests to use 3% milk for the blocking procedure. My questions are: Is it known that casein phosphorylation occurs also on tyrosine residues? Should I expect to see high background if milk is used for blocking? Is it possible that the producer gives a wrong technical suggestion?