In this task, you are given a sentence. You are expected to recognize the name of gene or protein. Although there might be several correct answers, you need to write one of them.

Let me give you an example: Sox - 4 is important for very early B - cell differentiation , while TCF - 1 / LEF - 1 play a crucial role in early thymocyte development 
The answer to this example can be: TCF - 1
Here is why: The gene, TCF - 1 has been tagged as protein since it plays a crucial role in early thymocyte development.

OK. solve this:
 Here we show that STAT5A can serve as a functional substrate of Bruton ' s tyrosine kinase ( BTK ) .
Answer:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
tyrosine kinase