A family of ribosomal protein s6 kinases that are considered the major physiological kinases for RIBOSOMAL PROTEIN S6. Unlike ribosomal protein s6 kinases, 90KDa the proteins in this family are sensitive to the inhibitory effects of RAPAMYCIN and contain a single kinase domain. They are referred to as 70kDa proteins, however alternative splicing of mRNAs for proteins in this class also results in 85kDa variants being formed.