Bcl-2 family proteins regulate apoptosis in animals. This protein family 
includes several homologous proteins and a collection of other proteins lacking 
sequence similarity except for a Bcl-2 homology (BH)3 motif. Thus, membership in 
the Bcl-2 family requires only one of the four BH motifs. On this basis, a 
growing number of diverse BH3-only proteins are being reported. Although 
compelling cell biological and biophysical evidence validates many BH3-only 
proteins, claims of significant BH3 sequence similarity are often unfounded. 
Computational and phylogenetic analyses suggest that only some BH3 motifs arose 
by divergent evolution from a common ancestor (homology), whereas others arose 
by convergent evolution or random coincidence (homoplasy), challenging current 
assumptions about which proteins constitute the extended Bcl-2 family.