I found it mentioned at precision-camera.com
ISO Sensitivity of 100 - 12800 - Select from the broad sensitivity range of ISO100-12800 for shooting at a full 12-megapixel resolution. The FUJIFILM X20 also lets you set the AUTO ISO range (up to ISO 3200) and the shutter speed limit. For quick access to ISO settings, you can assign this function to the Fn or Q Button.