GoogleMaps v3 API Create only 1 marker on click

I am successfully creating a marker on click however, using the following code, I get a new marker with every click, I only ever want one marker added and if someone clicks more than once I would like it to move the existing marker to the new position Can anyone help here is the code

 function placeMarker(location) {
        var clickedLocation = new google.maps.LatLng(location);
        var marker = new google.maps.Marker({
            position: location,
            map: map
        });
    }

google.maps.event.addListener(map, 'click', function(event) {
        placeMarker(event.latLng);