The regulation of gene expression during thymocyte development provides an ideal 
experimental system to study lineage-commitment processes. In particular, 
expression of the CD4, CD8A and CD8B genes seems to correlate well with the 
cell-fate decisions that are taken by thymocytes, and elucidating the molecular 
mechanisms that underlie the differential expression of these genes could reveal 
key events in differentiation processes. Here, we review examples of how gene 
cis elements (such as promoters, enhancers and locus control regions) and trans 
elements (such as transcription factors, chromatin-remodelling complexes and 
histone-modification enzymes) come together to orchestrate a finely tuned 
sequence of events that results in the complex pattern of CD4, CD8A and CD8B 
gene expression that is observed during thymocyte development.