Figure 2

Size distribution of protein families in relation to the time of their discovery. The x-axis represents the time of discovery of the founding member of a family; the y-axis represents frequency (on a logarithmic scale); each circle represents the number of protein families corresponding to the value on the y-axis; and the area of each circle corresponds to family size. It is notable that some of the largest families were founded early, but large families are still being discovered. Recently discovered small families (upper right) are expected to grow with better sampling of protein space.