ApoB100 is found in lipoproteins originating from the liver (VLDL, IDL, LDL). Importantly, there is one ApoB100 molecule per hepatic-derived lipoprotein. Hence, using that fact, one can quantify the "number" of lipoprotein particles by noting the total ApoB100 concentration in the circulation. Since there is one and only one ApoB100 per particle, the number of particles is reflected by the ApoB100 concentration. The same technique can be applied to individual lipoprotein classes (e.g. LDL) and thereby enable one to "count" them as well.