RadioButton IsChecked binding in View Model

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XAML

Binding

I know this topic has been discussed many times, but I still cannot get it work. I have 3 RadioButtons that are put in a StackPanel; I am trying to bind their IsChecked to some variable in View Model. Here is what I did:

in XAML file in View:

 <RadioButton Name="rbExpReview"
              IsChecked="{Binding Path=rbExpReviewIsChecked, Mode=TwoWay}"
              Foreground="White">
                   <RadioButton.Content>
                       <Label Content="Experiment Review"
                              HorizontalContentAlignment="Center"
                              VerticalContentAlignment="Center" />
                   </RadioButton.Content>
</RadioButton>

In View Model, I have declared a variable:

_rbExpReviewIsChecked = true;

and the property that responds to IsChecked of the RadioButton:

public bool rbExpReviewIsChecked
       {
           get
           {
               return _rbExpReviewIsChecked;
           }
           set
           {

               _rbExpReviewIsChecked = value;
               OnPropertyChanged("rbExpReviewIsChecked");
           }
       }

However, when I check and uncheck the button, there is no response in the View Model code (break point is not hit.) So I am wondering what is the problem with my binding? Or anything else? I am new to XAML, so any pointer is appreciated. Nick