Poser figures almost always come pre-rigged, although not usually with breast bones, but often with breast morph targets.

In the Marketplace here you can find rigging and movement morphs for the DAZ figures that will meet your needs.  I think there's one called something like "breast gravity system".  and 'natural gravity morphs'.

There is another elsewhere for DAZ V4 called "Natural Bounce"-if you google it you'll find it.

God Luck,

Rod

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