 There is an unknown number, probably thousands, of distinct cell types present in the developing embryo, including progenitor cell lines which are only temporarily present and which disappear either by differentiating into the permanent somatic cell types which make up the tissues of the infant's body at birth (or into other progenitor cell lines), or by undergoing the programmed cell death process known as apoptosis.Each cell type is defined by which genes are characteristically active in that cell type.A particular gene in a cell's genome codes for the production of a particular protein, that is, when that gene is turned on (active), the protein coded for by that gene is produced and present somewhere in the cell.