Hi there I recently start working with some archaic samples (aDNA), specifically two high coverage samples of Neanderthal and Denisova.

The latter has on single BAM file aligned to GRCh37 and already sorted; unfortunately, Neanderthal has five BAMs which were not sorted and needed to be merged...

So, I first proceeded to sort them — with -n to have the files ordered by read name — and then attempt to merge them with samtools; however, I was getting prompted the following:

[bam_translate] RG tag "L9302" on read "SN7001204_0130_AC0M6HACXX_PEdi_SS_L9302_L9303_1:1:1101:1050:3313" encountered with no corresponding entry in header, tag lost. Unknown tags are only reported once per input file for each tag ID.
[bam_translate] RG tag "L9105" on read "NIOBE_0139_A_D0B5GACXX:6:1101:1227:3642" encountered with no corresponding entry in header, tag lost. Unknown tags are only reported once per input file for each tag ID.
[bam_translate] RG tag "L9198" on read "SN928_0068_BB022WACXX:1:1101:1094:3181" encountered with no corresponding entry in header, tag lost. Unknown tags are only reported once per input file for each tag ID.
[bam_translate] RG tag "L9199" on read "SN928_0073_BD0J78ACXX:1:1101:1249:195244" encountered with no corresponding entry in header, tag lost. Unknown tags are only reported once per input file for each tag ID.
[bam_translate] RG tag "L9303" on read "SN7001204_0130_AC0M6HACXX_PEdi_SS_L9302_L9303_1:1:1101:1050:9131" encountered with no corresponding entry in header, tag lost. Unknown tags are only reported once per input file for each tag ID.

After a bit of research, I though the problem could have been that sorting and merging alone would have caused this; hence, following one post I thought to re-head the five files with the following:

samtools view -H nea_<#>.bam | grep -v "^@RG" | samtools reheader - nea_<#>.bam > nea_<#>_reheaded.bam

Despite doing so, though, the problem persist and as it is not an operation I do routinely (let alone on aDNA) I really cannot pinpoint to what is the source of it... Also, is it something concerning since the next step will be extracting the R1 & R2 from this BAM. Let me know, thanks in advance!