Virtual importance in the projection plot (VIP). The VIP shows the role of identified peptides in the separation of the four age and EE2 exposure PLS/DA classes (ed12C, ed12E, ed17C, ed17E). The preprotachykinin-1 neuropeptide K (NPK) is the top candidate among identified peptides for being affected by both age and EE2 exposure. Values above one are the most relevant for the separation of all classes. Unidentified peptides are not shown. Error bars designate confidence intervals derived by jack knifing.