Although many factors are known to be necessary for development of the immune cells called T cells, there have remained a number of mysteries about how these factors actually work. It is known that blood-cell precursors begin to develop into T cells but for some time preserve the ability to change paths and develop into some other cell types instead. The decision to develop as a T cell is only finalized during a specific commitment process. Until now, it has been unknown what protein factors cause blood-cell precursors to become committed to develop into T cells. This paper identifies the regulatory factor called Bcl11b, a gene expression control protein, as a crucial and unique factor that is needed to make T-cell commitment possible. This factor is turned on for the first time in the precursors when they are ready to become committed and then remains on through the life of the T cells. This paper shows that Bcl11b is the critical factor that is needed to enable the cells to turn off stem-cell genes, give up immature-cell growth properties, and give up the ability to change developmental path.