An enzyme activity assay was performed to determine whether Tat and morphine exposure alters CK2 enzyme kinetics. CK2 activity was determined in cytoplasmic extracts using the CycLex CK2 activity kit (# CY-1170, MBL Life Science, CycLex Co. Ltd, Nagano, Japan) per the manufacturer's instructions and previous publications (Jung et al., 2014 (link); Lee et al., 2014 (link); Oinuma et al., 2010 (link)). Briefly, 5 µg samples obtained from lysates of cells co-treated with Tat and morphine, or vehicle-treated control striatal neurons (receiving no experimental treatments) were incubated in wells pre-coated with a synthetic CK2-specific peptide with a serine 46 moiety that can be phosphorylated by CK2. A peroxidase-coupled monoclonal antibody was used to selectively detect phosphorylation of the serine 46 moiety of CK2 via the conversion of tetramethylbenzidine (TMB) to a chromogenic reaction product. The TMB reaction product was read at an absorbance of 450 nm on a PHERAstar FS plate reader (BMG LabTech; Cary, NC). For this study, reaction/incubation times used were 0 min, 20 min, and 40 min. Due to the unavailability of a CK1 enzyme activity kit and since CK1 levels were not changed in HIV- or Tat-exposed tissues in the present study, further evaluation of CK1 activity did not seem warranted.