+---------SUBJ:V-N--------+                         
      +----COMP:N-N(of)---+     +---COMP:V-N(in)---+      
      |            +MOD_AT+     |         +MOD_ATT:+      
      |            |      |     |         |        |      
 Expression of __NODE__ mRNA occurs in __SP__ leukocytes .
COMP:N-N(of) (expression,mRNA)
MOD_ATT:N-ADJ (mRNA,__NODE__)
SUBJ:V-N (occur,expression)
COMP:V-N(in) (occur,leukocytes)
MOD_ATT:N-ADJ (leukocytes,__SP__)