CD34+ CD38- cont'd

OK,

I have just come across a paper: Batard et a,l Cytometry 48:97-105 (2002) about FRET 
between PE and APC. 
In the conclusion they state that FRET between PE and APC can be high enough to 
generate nonspecific labelling in a channel that is generally associated with PerCP and 
PECY5 in multicolour protocols. 

Could this be the problem?
 although I do not understand how the FRET occurs after cell sorting if the cells are 
selected in a PECY5 negative gate.
I will of course check this with an FMO ctrl (i.e. minus the PECY5 label).
The fluorescent labels I am using are: Fitc, PE, PECY5 or PerCPCY5.5, PECY7, APC, 
APCCY7, Hoechst.

regards
Ann