From a to a is a short supplemental textbook that uses control of yeast mating type as a model for many aspects of cell determination in general. Topics covered include gene silencing; genetic recombination; differentiation; combinatorial gene regulation; mRNA transport to establish asymmetric cell division; signal transduction; evolution of genetic networks; and various aspects of cell biology, including action of cytoskeleton and bud site selection. The book has a foreword by Mark Ptashne, author of A Genetic Switch.