There are 19724 sequences in the database. I could have used length(mature) instead of hard coding the number. You also need to initialise out as a variable first.
> for (x in c(1:19724)) out <- c(out, GC(mature[[x]])) > hist (out)
So I am happy with this as it is normally distributed and so I do not need to take any special care in the GC/AT content for my training sets.