var subModel = Backbone.Model.extend({ constructor:function(){ //extend this constructor, not Model this.foo = 'bar'; } }); myModel = new subModel(); console.log(myModel.foo); //logs bar /*cid is an instance property set by the Model constructor but logs undefined because we created our own constructor*/ console.log(myModel.cid); /*the cid property is an instance property set by Model() and we didn't use Model, we used a custom constructor so myModel does not have a cid property*/