The present invention relates to Shiga toxin effector polypeptides with reduced antigenic and/or immunogenic potential. Immunogenicity can be a limitation for the repeated administration to mammals of proteins and polypeptides derived from Shiga toxins. The Shiga toxin effector polypeptides of the present invention have uses as components of therapeutics, diagnostics, and immunization materials. The cytotoxic proteins of the present invention have uses for selective killing of specific cell types and as therapeutics for the treatment of a variety of diseases, including cancers, immune disorders, and microbial infections. The proteins of the present invention also have uses for detecting specific cell types, collecting diagnostic information, and monitoring the treatment of a variety of diseases, such as, e.g., cancers, immune disorders, and microbial infections.