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4 

MAYO COLLABORATIVE SERVICES v. PROMETHEUS 

LABORATORIES, INC.
 
Opinion of the Court 

at 14) (“[T]he prohibition against patenting abstract ideas
‘cannot be circumvented by attempting to limit the use of 
the  formula  to  a  particular  technological  environment’  or
adding ‘insignificant postsolution activity’ ” (quoting Diehr, 
supra, at 191–192)). 

We  find  that  the  process  claims  at  issue  here  do  not
satisfy  these  conditions.  In  particular,  the  steps  in  the
claimed  processes  (apart  from  the  natural  laws  them­
selves)  involve  well-understood,  routine,  conventional 
activity  previously engaged in by researchers in the field. 
At  the  same  time,  upholding  the  patents  would  risk  dis­
proportionately  tying  up  the  use  of  the  underlying  nat- 
ural  laws,  inhibiting  their  use  in  the  making  of  further 
discoveries. 

I 

A 

The  patents  before  us  concern  the  use  of  thiopurine 
drugs  in  the  treatment  of  autoimmune  diseases,  such  as
Crohn’s  disease  and  ulcerative  colitis.    When  a  patient 
ingests  a  thiopurine  compound,  his  body  metabolizes  the
drug,  causing  metabolites  to  form  in  his  bloodstream. 
Because  the  way  in  which  people  metabolize  thiopurine 
compounds  varies,  the  same  dose  of  a  thiopurine  drug 
affects different people differently, and it has been difficult
for doctors to determine whether for a particular patient a 
given dose is too high, risking harmful side effects, or too 
low, and so likely ineffective. 

At  the  time  the  discoveries  embodied  in  the  patents 
were  made,  scientists  already  understood  that  the  levels
in  a  patient’s  blood  of  certain  metabolites,  including,  in 
particular,  6-thioguanine  and  its  nucleotides  (6–TG)  and 
6-methyl-mercaptopurine  (6–MMP),  were  correlated  with
the  likelihood  that  a  particular  dosage  of  a  thiopurine 
drug  could  cause  harm  or  prove  ineffective.    See  U. S. 
Patent  No. 6,355,623,  col.  8,  ll.  37–40,  2  App.  10.  (“Previ­