For a subset of cutaneous melanoma patients with day 21 data we were able to measure unequivocal CMV serology. a, CMV seropositivity is associated with a depletion of small clones (ratio <1 for clones <0.05% repertoire) and increased numbers of large clones (>2% repertoire) (n=68 left panel, 53 right panel). b, Patients seropositive for CMV demonstrated significantly reduced CD8+ TCR diversity at day 21 (measured on TRB CDR3) (n=53, two-sided T-Test). c, Despite significant differences in diversity from CMV there was no difference in diversity between day 21 samples from progressing and responding patients (n=53, two-sided T-test). d, There is no association between CMV serology and number of large clones at day 21 (n=53, two-sided T-Test) For all boxplots lower and upper hinges of box represent 25-75th percentiles, central line the median and the whiskers extend to largest and smallest values no greater than 1.5x interquartile range.