Accession numbers associated with the sequences analyzed are listed in the Methods section. M1, M2, M3, M7 and M8 refer to maximum likelihood models of ω ratios, and -2(M2 vs M1), -2(M3 vs M1) and -2(M8 vs M7) indicate the negative of two times the log likelihood difference between the selection and neutral models compared. P values for the test statistics are shown in parentheses. For M8, p1 is the proportion of positively selected sites, p0 is the proportion of sites not under positive selection, ω is the dN/dS ratio for the selected sites, and β(p, q) describes the beta distribution function. Positively selected ULBP sites are presented according to their numbered positions in the ULBP1 preprotein sequence. Posterior probabilities for positively selected sites are represented in normal text (probability > 0.90), bold text (probability > 0.95), and italicized bold text (probability > 0.99).