In summary, it can be argued that the understanding of eukaryotic rRNA processing is no less important than the understanding of mRNA maturation, since the capacity of a cell to carry out protein synthesis is controlled, in part, by the abundance of ribosomes. Processing of pre-rRNA is highly regulated, involving many cellular components acting either alone or as part of a complex. Some of these components are directly involved in the modification and cleavage of the precursor rRNA, while others direct the packaging of the rRNA into ribosome subunits. As is the case for pre-mRNA processing, snoRNPs are clearly involved in eukaryotic rRNA processing, and have been proposed to assemble with other proteins into at least one complex called a "processosome" (17), which carries out the ordered processing of the pre-rRNA and its assembly into ribosomes. The formation of a processing complex clearly makes possible the regulation required to coordinate the abundance of ribosomes with the physiological and developmental changes of a cell. It may be that eukaryotic rRNA processing is even more complex than pre-mRNA maturation, since pre-rRNA undergoes extensive nucleotide modification and is assembled into a complex structure called the ribosome. Undoubtedly, features of the eukaryotic rRNA-processing pathway have been conserved evolutionarily, and the genetic approach available in yeast research (6) should provide considerable knowledge that will be useful for other investigators working with higher eukaryotic systems. Interestingly, it was originally hoped that the extensive work and understanding of bacterial ribosome formation would provide a useful paradigm for the process in eukaryotes. However, although general features of ribosome structure and function are highly conserved between bacterial and eukaryotic systems, the basic strategy in ribosome biogenesis seems to be, for the most part, distinctly different. Thus, the detailed molecular mechanisms for rRNA processing in each kingdom will have to be independently deciphered in order to elucidate the features and regulation of this important process for cell survival.