by subtype = by subtype (the scientists can identify GABA neurons very specifically)?

Plus, what is the subtype?

Context:

CSHL Professor Z. Josh Huang and colleagues in his neuroscience lab have spent portions of the last five years working on a project to comprehensively label GABA neurons. The results of their highly time-consuming labors are described in a paper appearing Sept. 22 in the journal Neuron. The paper is likely to be influential in the neuroscience community since it describes the creation of different lines of mice expressing genetic triggers that enable GABA neurons to be identified very specifically, by subtype, and to be tracked and manipulated in real time in living animals.

More:

http://www.sciencedaily.com/releases/2011/09/110921132342.htm