5'-pApA is a linearized form of cyclic di-AMP, a bacterial second messenger that activates the host innate immune system through stimulator of interferon genes (STING).1,2,3,4It is a metabolite of cyclic di-AMP formedviahydrolysis by various phosphodiesterases (PDEs).55'-pApA is intended for use as a negative control for cyclic di-AMP signaling. 1.Burdette, D.L., Monroe, K.M., Sotelo-Troha, K., et al.STING is a direct innate immune sensor of cyclic-di-GMPNature478(7370)515-518(2011) 2.Parvatiyar, K., Zhang, Z., Teles, R.M., et al.DDX41 recognizes bacterial secondary messengers cyclic di-GMP and cyclic di-AMP to activate a type I interferon immune responseNat. Immunol.13(12)1155-1161(2012) 3.Woodward, J.J., Iavarone, A.T., and Portnoy, D.A.c-di-AMP secreted by intracellular Listeria monocytogenes activates a host type I interferon responseScience328(5986)1703-1705(2010) 4.Witte, C.E., Whiteley, A.T., Burke, T.P., et al.Cyclic di-AMP is critical for Listeria monocytogenes growth, cell wall homeostasis, and establishment of infectionmBio4(3)e00282-00213(2013) 5.Fahmi, T., Port, G.C., and Cho, K.H.c-di-AMP: An essential molecule in the signaling pathways that regulate the viability and virulence of gram-positive bacteriaGenes (Basel)8(8)197(2017)