Martin,

Thanks for the quick response!

I tried to set the tag parameter, but NULL is returned. Here is the code and sessionInfo. 

pram1 <- ScanBamParam(
    tag=c( "MD", "XN", "AS"),
    what=c("rname", "strand", "pos", "qwidth",  "cigar", "mapq", "qual")
)
bam <- scanBam(bamPaths, index = bamIndex, pram = pram1)
str(bam[[1]][["tag"]])
NULL

I also tried with the example code in ScanBamParam with MD added to the tag, but for some reason, NULL is set in the MD field.

fl <- system.file("extdata", "ex1.bam", package="Rsamtools", mustWork=TRUE)
p4 <- ScanBamParam(tag=c("NM", "H1", "MD"), what="flag")
bam4 <- scanBam(fl, param=p4)     
str(bam4[[1]][["tag"]])
List of 3
 $ NM: int [1:3307] 0 0 0 5 0 1 0 1 0 1 ...
 $ H1: int [1:3307] 0 0 0 0 0 1 0 1 0 0 ...
 $ MD: NULL

Best regards,

Julie

sessionInfo()
R Under development (unstable) (2016-06-30 r70858)
Platform: x86_64-apple-darwin12.5.0 (64-bit)
Running under: OS X Mountain Lion 10.8.5
locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats4    parallel  stats     graphics  grDevices utils     datasets 
[8] methods   base     

other attached packages:
[1] Rsamtools_1.27.11     Biostrings_2.43.2     XVector_0.15.0       
[4] GenomicRanges_1.27.18 GenomeInfoDb_1.11.6   IRanges_2.9.14       
[7] S4Vectors_0.13.5      BiocGenerics_0.21.2  

loaded via a namespace (and not attached):
[1] zlibbioc_1.21.0    tools_3.4.0        BiocParallel_1.9.4 bitops_1.0-6