We next examined the frequency of poly A and poly U tracts in the hMPV sequence spanning nt4536 to nt5623, to determine whether there is a bias between insertions of A or U. This region contains 14 poly A tracts and 3 poly U tracts with 4 or more contiguous A or U residues, respectively. Among the 15 cloned RT-PCR products amplified from the genomic RNA, 26 incidences of inserted A and 1 of inserted U were observed ( Figure 3C ). Thus, the data suggest a strong bias for insertions of A.