WP7 Binding a font in Application.Resources of App.xaml

Question

In App.xaml

<Application.Resources>

Let's say I have

<FontFamily x:Key="SomeFontName">Segoe WP</FontFamily>

How to make

<FontFamily x:Key="SomeFontName">{BINDING ?????}</FontFamily>

Or may be there is another way of setting a FontFamily with Binding in App.xaml?