Figure 1.

Schematic representation of sequential activation of bone master gene promoters used for Cre expression or gene overexpression. During bone development there is a successive expression of master genes that provide the special characteristics of bone cells. For example, chondroprogenitor cells express the Dermo1/Twist2 promotor, if fused with Cre protein and expressed in the cell, flox-tagged genes will be removed and the complete cell linage will be affected. However, if Cre is controlled by a late gene such as DMP1, which is mainly expressed in osteocytes, flox-tagged genes will be primarily removed in osteocytes and not preceding cell lineage types, such as the osteoblasts. The promoters mentioned in this review and the differentiation state for their activation are summarized in the table below.