The building blocks of FACS are 46 action units  (AUs) such as a wrinkling of the nose, a puffing of the cheeks, dilation of the nostrils, squinting, and so on.
There are forty-six action units, like squinting and wrinkling the nose, that comprise the basic elements of FACS.
The action units of FACS involve only body movements, not facial expressions like squinting.