In principle, the SARS-CoV-2/HCOV-HKU1 S cross-reactive memory B cells could be pre-existing in the COVID donors and show cross-reactivity with SARS-CoV-2 or originate from the SARS-CoV-2 infection and show cross-reactivity with HCoV-HKU1 S protein. This evidence concerns the gene PROS1 and COVID-19.