Is there any value in initializing a single model for the `variant`? This is being used a couple times above. The `Variant` model could define all the sub-{models, collections} for the nested structures such cohorts, etc. This makes it a bit more clear what embedded data is supported and enables implementing sub-document parsing on nested models/collections. For example:

```javascript
var Variant = Backbone.Model.extend({
    constructor: function(attrs, options) {
        // Ensure to parse..
        options = _.defaults({parse: true}, options);

        this.cohorts = new Backbone.Collection();
        this.frequencies = new Backbone.Collection();
        // etc..
        
        Backbone.Model.prototype.constructor.call(this, attrs, options);
    },
    parse: function(attrs) {
        this.frequencies.reset(attrs.frequencies);
        this.cohorts.reset(attrs.cohorts);
        // etc..
        return attrs;
    }
});
```