Well, remember, from basic biology, calcium is used by muscle cells. And so what that implies is the calcium Atpas must be found within muscle cells. And that's exactly right. In the membrane of the sarcoplasmic reticulum, the specialized type of endoplasm reticulum is found inside our muscle cells. Within the membrane we find these calcium ATPases. In fact, calcium Atpas makes up up to 80% of the protein content of the sarcoplasm reticulum.