Hi, i was trying to extract unmapped reads from a BAM alignment, using following commands

1) samtools view -f 4 bamfile >temp1

2) samtools view -f 8 bamfiles >temp2

3) samtools merge -u - temp[12] | samtools sort -n > unmappedBAM

But when i checked the unmappedBAM files using samtools flagstat, i get 27% mapped reads in it, which are singletons.

1578683 + 0 in total (QC-passed reads + QC-failed reads)

28683 + 0 secondary

0 + 0 supplementary

0 + 0 duplicates

428595 + 0 mapped (27.15% : N/A)

1550000 + 0 paired in sequencing

775000 + 0 read1

775000 + 0 read2

0 + 0 properly paired (0.00% : N/A)

0 + 0 with itself and mate mapped

399912 + 0 singletons (25.80% : N/A)

0 + 0 with mate mapped to a different chr

0 + 0 with mate mapped to a different chr (mapQ>=5)

I would like to know if singletons are to be kept in the unmappedBAM file or should i remove these as well. I plan to re assemble the unmapped genes, so in that regard I seek suggestion.