Of the estimated 20,000 genes humans have, only a fraction are turned on at any given time. It depends on the cell's needs, which can change by the minute or hour.
Determining what those genes are doing means using tools that can manipulate their status on similarly short timescales and that is now possible with technology developed by the Eli and Edythe L. Broad Institute of Harvard and MIT. It can rapidly start or halt the expression of any gene simply by shining light on the cells.
The advance in optogenetics, which uses proteins that change their function in response to light, is possible because researchers adapted light-sensitive proteins to either stimulate or suppress the expression of a specific target gene almost immediately after the light comes on.