During organogenesis, cell fates are specified by the expression of particular gene sets. Development of the fore- and hindlimbs is a good model to understand how cell identity and organ morphology are determined. The fore- and hindlimbs originate from the lateral plate mesoderm and initially form a simple budding structure. Although morphology of the fore- and hindlimbs is distinct, most developmental genes are expressed with similar patterns in the both limb buds. The molecular basis that determines differential development of the fore- and hindlimbs is still not fully understood. In this class, we will see two paralogous transcription factors, T-box 4 and 5, which are exclusively expressed in the fore- and hindlimb buds, and discuss specification of organ identity.