Researchers at the California Institute of Technology (Caltech) have outlined the developmental process that ensures certain stem cells will become T cells, immune system cells that help fight pathogens. The researchers observed multipotent hematopoietic precursor cells, which express a variety of genes and differentiate into blood-cell types. They identified all the genes that play a role in transforming precursor cells into committed T cells and noted when in the developmental process they each turn on. Five stages involved in molecular events that yield a T cell were studied: two before commitment, a commitment stage, and then two following commitment. Ultra-high-throughput DNA sequencing was used to identify when major changes in gene expression occur during development. Also, an in vitro tissue-culture system allowed the team to mass-produce synchronized early T-cell precursors.