Illustration of antisense hybridization imaging approach. Small radioactive antisense oligodeoxynucleotides (RASONs) are used to target a small portion of a chosen mRNA. If sufficient levels of mRNA are present, it may be possible to retain the RASONs only in those cells expressing the mRNA. Efflux would occur in other cells if non-specific interactions can be minimized. Because radiolabeling chemistry can be made independent of the RASON sequence, many different types of mRNA can potentially be targeted.