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10  MAYO COLLABORATIVE SERVICES v. PROMETHEUS 

LABORATORIES, INC. 
Opinion of the Court 

operators  about  his  basic  law  and  then  trusting  them  to
use it where relevant).

Third,  the  “determining”  step  tells  the  doctor  to  deter­
mine  the  level  of  the  relevant  metabolites  in  the  blood, 
through  whatever  process  the  doctor  or  the  laboratory
wishes to use.  As the patents state, methods for determin­
ing  metabolite  levels  were  well  known  in  the  art.    ’623 
patent,  col.  9,  ll.  12–65,  2  App.  11.    Indeed,  scientists 
routinely measured metabolites as part of their investiga­
tions into the relationships between metabolite levels and 
efficacy and toxicity of thiopurine compounds.  ’623 patent,
col.  8,  ll.  37–40,  id.,  at  10.  Thus,  this  step  tells  doctors
to engage in well-understood, routine, conventional activity 
previously engaged in by scientists who work in the field. 
Purely “conventional or obvious” “[pre]-solution activity” is
normally  not  sufficient  to  transform  an  unpatentable  law 
of  nature  into  a  patent-eligible  application  of  such  a  law. 
Flook,  437  U. S.,  at  590;  see  also  Bilski,  561  U. S.,  at  ___ 
(slip  op.,  at  14)  (“[T]he  prohibition  against  patenting  ab­
stract ideas ‘cannot be circumvented by’ . . . adding ‘insig­
nificant  post-solution  activity’ ”  (quoting  Diehr,  supra,  at 
191–192)).

Fourth, to consider the three steps as an ordered combi­
nation  adds  nothing  to  the  laws  of  nature  that  is  not  al­
ready  present  when  the  steps  are  considered  separately. 
See Diehr, supra, at 188 (“[A] new combination of steps in 
a process may be patentable even though all the constitu­
ents  of  the  combination  were  well  known  and  in  common 
use  before  the  combination  was  made”).    Anyone  who
wants  to  make  use  of  these  laws  must  first  administer  a 
thiopurine  drug  and  measure  the  resulting  metabolite
concentrations,  and  so  the  combination  amounts  to  noth­
ing  significantly  more  than  an  instruction  to  doctors  to 
apply the applicable laws when treating their patients. 

The upshot is that the three steps simply tell doctors to
gather  data  from  which  they  may  draw  an  inference  in