HPRT1 and COVID-19: A refined linear regression analysis of the proteomic hits (FDR < 0.05) further identified 23 upregulated proteins, of which the top 9 are the same as described above, and revealed 2 downregulated proteins in COVID-19+ patient urine, hypoxanthine phosphoribosyltransferase 1 (HPRT1) and CD5 molecule–like (CD5L) (Supplemental Figure 6A).