An equivalence group is a set of unspecified cells that have the same developmental potential or ability to adopt various fates. Our current understanding suggests that equivalence groups are limited to cells of the same ancestry, also known as sibling cells. Often, cells of an equivalence group adopt different fates from one another. Equivalence groups assume various potential fates in two general, non-mutually exclusive ways. One mechanism, induction, occurs when a signal originating from outside of the equivalence group specifies a subset of the naïve cells. Another mode, known as lateral inhibition, arises when a signal within an equivalence group causes one cell to adopt a dominant fate while others in the group are inhibited from doing so. In many examples of equivalence groups, both induction and lateral inhibition are used to define patterns of distinct cell types. Cells of an equivalence group that do not receive a signal adopt a default fate. Alternatively, cells that receive a signal take on different fates. At a certain point, the fates of cells within an equivalence group become irreversibly determined, thus they lose their multipotent potential. The following provides examples of equivalence groups studied in nematodes and ascidians. Vulva Precursor Cell Equivalence Group Introduction A classic example of an equivalence group is the vulva precursor cells (VPCs) of nematodes. In Caenorhabditis elegans self-fertilized eggs exit the body through the vulva. This organ develops from a subset of cell of an equivalence group consisting of six VPCs, P3.p-P8.p, which lie ventrally along the anterior-posterior axis. In this example a single overlying somatic cells, the anchor cell, induces nearby VPCs to take on vulva fates 1° (P6.p) and 2° (P5.p and P7.p). VPCs that are not induced form the 3° lineage (P3.p, P4.p and P8.p), which make epidermal cells that fuse to a large syncytial epidermis (see image). The six VPCs form an equivalence group because all of the six cells are competent to take on any of the available fates (1°, 2°, and 3°) dependent on their proximity to the anchor cell. Ablation experiments indicate that all VPCs are able to adopt vulva fates. For example, if the P6.p cell that normally becomes 1° is ablated then the VPC closest to the anchor cell, either P5.p or P7.p, assumes the 1° fate. Furthermore, if all VPCs are destroyed except the most anterior P3.p cell then the anchor cell designates this cell the 1°