Final answer:
The characteristic transcription factor of Treg cells is FOXP3, which is crucial for their suppressive function and development.
Step-by-step explanation:
The transcription factor characteristic of Treg cells is FOXP3. Regulatory T cells (Treg), or suppressor T cells, play a crucial role in maintaining immune homeostasis by suppressing other T cell immune responses. The presence of FOXP3 is a key feature that distinguishes Treg cells from other T cell subsets and is essential for their suppressive function and development. Transcription factors like FOXP3 are crucial for regulating the expression of genes and, ultimately, determining the presence of particular proteins within a cell.