Does anyone know of a stict S phase marker? - (Jun/24/2015 )

I am looking for a protein that has expression restricted to S phase only. Not sure if one exists. More specifically, high expression in S phase but no expression or very low expression in G1, G2/M. I want such a marker so I can identify S phase cells during immunofluorescence. Some people have recommended Cyclin A because of its S phase expression however it also expressed during G2. 

Thanks!

Have you checked Proliferating cell nuclear antigen (PCNA) ?

Kisielewska J et al. 2005

Thanks for the reference! I am aware of PCNA as a proliferation marker. However, I never considered it for an S phaser indicator. It does have peak expression at S phase but it also has low expression in late s and during G2. This is the case for many S phase proteins. Regardless of the S phase protein, I think the best option is too only consider cells with highest PCNA expression to be in S phase. I am not sure if this expression pattern is deregulated in cancer cells.