There were three possible explanations. The first one was that we were doing something wrong and that part of the mitochondria failed to come down with the mitochondrial fraction or that we removed them when we decanted and came down with the microsomes. Second possibility: it just happens to be so that there is some acid phosphatase in the mitochondria and some acid phosphatase in the microsomes; and many other investigators had just taken that as a reasonable explanation for other findings of the same kind. And being simple minded and single minded I thought, well, there could be a third explanation: namely that acid phosphatase is not present in the mitochondria, is not present in the lysosomes, but is present in another particle that happens to be such that two thirds of it comes down with the mitochondria and one third with the microsomes. And... so in order to distinguish between these three explanations, we started... well, first of all we compared the distribution of our enzymes with those of enzymes of known localisation and here's where we come back to Hogeboom and Schneider, in fact even to Claude, because that was already, to some extent, established by... by him earlier. The respiratory enzyme... the enzyme responsible for the uptake utilisation of oxygen in the liver and elsewhere was shown to be exclusively associated with the mitochondria; this had been... cytochrome oxidase as it is now known, the so-called 'atmungsenzyme' respiratory enzyme, as it was named by Otto Warburg, who came before in this story as the mentor of my mentor, Hugo Theorell.