3501
Tumor cells express the glycolytic regulator pyruvate kinase subtype M2 ( M2 - PK ) , which can occur in a tetrameric form with high affinity to its substrate phosphoenolpyruvate ( PEP ) and a dimeric form with a low PEP affinity .
 Tumor cells
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Anatomy 
Text: {0} 
Answer:
 Tumor cells