Different Genes - Different Functions
Although each cell contains the genetic potential to make all possible human proteins, cells use genes selectively.
Some housekeeping genes encode proteins needed for basic cell functions, and these genes stay active all the time in many cell types. More typically, though, a cell activates just the genes it needs at the moment and actively suppresses the rest. The unique selection of genes used by a cell gives that cell its character--making a brain cell, say, different from a bone cell.