\begin{tikzpicture}
	\tikzset{
		mstyle/.style={column sep=-\pgflinewidth,nodes={draw},font=\footnotesize},
		window/.style={draw,very thick,blue},
	}
	\matrix(m)[matrix of nodes,ampersand replacement=\&,mstyle]{
		A \& B \& C \& B \& A \& B \& C \\
	};
	\foreach \j/\k [count=\i] in {3/2,4/3,5/4,6/5,7/6}{
		\onslide<\k>{
			\draw[window](m-1-\i.north west)rectangle(m-1-\j.south east);
	}}
\end{tikzpicture}