Even though synthesis using ink-jet-based printing or electrochemistry is not intrinsically limited to DNA, there appear to be no examples in the literature of the use of these technologies beyond DNA. In contrast, there has been extensive innovation in the use of alternative building blocks and modifications in the context of the photolithographic approach. These include: peptides , peptide nucleic acid (PNA) , reverse (5′→3′) DNA synthesis (allowing, e.g., synthesis of arrays of reverse transcription primers for spatial transcriptomics) , ʟ-DNA , RNA , 2′OMe-RNA , 2΄-fluoroarabinonucleic acid (2΄F-ANA) , deoxyuridine , and dNaM , structural modifiers such as asymmetric branches , hexaethylene glycol (HEG) linkers , and strand crosslinkers , as well as direct fluorescence labeling via cyanine and fluorescein phosphoramidites.