Which of the following statements about endogenous synthesis of insulin are correct?
\[
P. \text{Insulin is synthesized as preproinsulin.} \quad Q. \text{Preproinsulin is converted to proinsulin.} \quad R. \text{Single-site cleavage of proinsulin eliminates C chain.} \quad S. \text{Mature insulin consists of disulfide-linked A and B chains.}
\]