That = ?

That = segments of DNA and gene-regulating proteins ?

Context:

Once the team had deciphered the genetic signals that guide the development of these cells, they were able to coerce human embryonic stem cells into developing in the same way. To initiate the metamorphosis, they created pores in the walls of the stem cell nuclei and slipped in segments of DNA and gene-regulating proteins called transcription factors that are associated with the neurons.