The present invention relates to compounds useful as inhibitors of ATR protein kinase. The invention also relates to pharmaceutically acceptable compositions comprising the compounds of this invention； methods of treating of various diseases, disorders, and conditions using the compounds of this invention； processes for preparing the compounds of this invention； intermediates for the preparation of the compounds of this invention； and methods of using the compounds in in vitro applications, such as the study of kinases in biological and pathological phenomena； the study of intracellular signal transduction pathways mediated by such kinases； and the comparative evaluation of new kinase inhibitors.The compounds of this invention have formula I:or a pharmaceutically acceptable salt, wherein the variables R1, R2, R3, and R4 are as defined herein.
