Many RNA-binding proteins of the nucleus and cytoplasm, including pre-mRNA-, mRNA-, snRNA-, and pre-rRNA-binding proteins, contain a putative RNA-binding domain of approximately 90 amino acids whose amino acid sequence is conserved from yeast to man. The most highly conserved motif within this RNA-binding domain is an octapeptide, termed the ribonucleoprotein consensus sequence {RNP-CSI, which is an identifying feature of this group of proteins. Frequently, these proteins contain several similar, but nonidentical, RNP-CS-type RNA-binding domains. All of these proteins also contain at least one auxiliary domain that is unique to each type of protein and most likely functions in protein-protein interactions. Many, if not all, of the RNP-CS-type proteins display binding preferences for specific RNA sequences, and several have been shown to interact with pre-mRNA sequences important for premRNA processing. Recent work has shown that the proteins encoded by several developmental loci in Drosophila and maize contain RNP-CS and, therefore, are most likely RNA-binding proteins. Here we provide an overview of the structural characteristics of these proteins and speculate on how the modular structure of RNP-CS-type RNA-binding proteins may facilitate their participation in pathways that regulate development at the post-transcriptional level.