Abstract: Provided are RNA polynucleotide sequences referred to as “EXO-Codes.” The RNA polynucleotides comprise one or more nucleotide sequences that facilitate preferential enrichment of secreted membranous vesicles that contain the EXO-Codes. Nucleotide motifs that contribute to these properties of the EXO-Codes are described. Modified eukaryotic cells comprising EXO-Codes are provided, and include lymphocytes such as T cells. EXO-Codes may comprise a cargo that provides a prophylactic or therapeutic effect. Exosome preparations comprising the EXO-codes are provided. Pharmaceutical compositions comprising EXO-Codes, expression vectors encoding them, and methods of using the pharmaceutical formulations are provided. The pharmaceutical compositions may comprise the EXO-Codes within membranous structures, such as exosomes. Cells that express or otherwise include the EXO-Codes are included, as are method for separating membranous structures that contain the EXO-codes from the cells.