Ribonuclease A is an endonuclease, which means that it can cut RNA in the middle of a strand. Other ribonucleases degrade RNA from the ends, such as the exosome that is involved in degradation of RNA inside cells. In ribonuclease A, the RNA chain fits in a groove and is attacked by a collection of acidic and basic amino acids. It is a voracious molecular shredder, cleaving after cytidine bases best, and after uridine bases about half as fast. It has been honed by evolution and is one of a select group of diffusion-limited enzymes that act as fast as substrates can get to them.