**6.1 FACS analysis and isolation of live cells based on intracellular protein expression**

One advantage of introducing a reporter into the gene locus of an intracellular protein is that one can then use the reporter mouse or cell line to identify and isolate live cells by Fluorescence Activated Cell Sorting (FACS) for in vitro applications. For example, we used the SMA-mCherry mice that we developed to isolate live cells expressing SMA from tissues during embryonic development (Fig. 3a), and demonstrated that the cells retain reporter expression after culture in vitro (Fig. 3b).

Fig. 3. Isolation of SMA positive cells by flow cytometry. (a) FACS analysis of E9.5 yolk sac endothelium reveals that a subset of endothelium expressed the mesenchymal marker SMA. (b) Expression of SMA was retained when the cells were cultured in vitro.
