+----------SUBJ:V-N---------+                 
      +----COMP:N-N(of)---+       +COMP:V-N(in)+    
      |           +MOD_ATT+       |       +MOD_+    
      |           |       |       |       |    |    
 Expression of __SP__ __NODE__ occurs in PB1 cells .
COMP:N-N(of) (expression,__NODE__)
MOD_ATT:N-ADJ (__NODE__,__SP__)
SUBJ:V-N (occur,expression)
COMP:V-N(in) (occur,cell)
MOD_ATT:N-N (cell,PB1)