MVVM WPF4.0 Checkbox checked programatically

I've searched all over for this - while there is PLENTY on binding a checkbox, I couldn't find anything on programmatically setting the box as checked, and the solutions that I thought I'd found didn't work.

The XAML:

<CheckBox Content="" Height="16" HorizontalAlignment="Right" Margin="0,297,118,0" Name="checkBox1" VerticalAlignment="Top"
                  Command="{Binding CheckCommand, Mode=TwoWay}" CommandParameter="{Binding IsChecked}">
        </CheckBox>

The VieModel:

private bool isChecked;
private ICommand checkCommand;
        
public bool IsChecked
        {
            get { return isChecked; }
            set
            {
                isChecked = value;
                OnPropertyChanged("IsChecked");
            }
        }

public ICommand CheckCommand
        {
            get 
            {
                if (checkCommand == null)
                    checkCommand = new CommandBase(i => Checkprocess(i), null);
                return checkCommand; 
            }
            set
            {
                checkCommand = value;
                OnPropertyChanged("CheckCommand");
            }
        }
public void Checkprocess(object sender)
        {
           //this DOES react when the checkbox is checked or unchecked
        }

public MyViwModel()
{
     IsChecked = true;
}

The value of IsChecked and isChecked DO become true, however the checkBox1 value remains unchecked.

What obvious thing am I missing here?

Tags: WPF, Checkbox, MVVM