transcriptToGenome(x, db, id = "name")

library(EnsDb.Hsapiens.v86)
## Restrict all further queries to chromosome x to speed up the examples
edbx <- filter(EnsDb.Hsapiens.v86, filter = ~ seq_name == "X")

## Below we map positions 1 to 5 within the transcript ENST00000381578 to
## the genome. The ID of the transcript has to be provided either as names
## or in one of the IRanges' metadata columns
txpos <- IRanges(start = 1, end = 5, names = "ENST00000381578")

transcriptToGenome(txpos, edbx)
## The object returns a GRangesList with the genomic coordinates, in this
## example the coordinates are within the same exon and map to a single
## genomic region.

## Next we map nucleotides 501 to 505 of ENST00000486554 to the genome
txpos <- IRanges(start = 501, end = 505, names = "ENST00000486554")

transcriptToGenome(txpos, edbx)
## The positions within the transcript are located within two of the
## transcripts exons and thus a `GRanges` of length 2 is returned.

## Next we map multiple regions, two within the same transcript and one
## in a transcript that does not exist.
txpos <- IRanges(start = c(501, 1, 5), end = c(505, 10, 6),
    names = c("ENST00000486554", "ENST00000486554", "some"))

res <- transcriptToGenome(txpos, edbx)

## The length of the result GRangesList has the same length than the
## input IRanges
length(res)

## The result for the last region is an empty GRanges, because the
## transcript could not be found in the database
res[[3]]

res