Reaction/mechanism for BCIP+NBT in immunoblotting

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	In the reaction between 5-bromo, 3-chloro, 4-indoyl phosphate 
suspension (BCIP) and nitroblue tetrazolium(NBT) which is catalyzed by the 
enzyme alkaline phosphatase, what is the reaction/mechanism for this 
reaction?  Does anybody know where I can find any journal references relating 
to this reaction?

				Sincerely,
				
			             Undergraduate desiring xtra credit...