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In the present study , we found that Ni ( 2 + ) exposure led to a time - and dose - dependent proliferation arrest and death in human leukemia HL - 60 cells .
 Chemical: Ni ( 2 + )
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: Ni ( 2 + )