[BioC] How to create nested data frames

[Ana Conesa]

Dear List,

This is more an R than a Bioconductor question but I cannot post at
the R list at the moment, so I apologize for using Bioconductor
instead.

I am trying to use the pls package to compute pls regression of gene
expression data on medical variables. I should provide my data as a
data.frame (NOT A LIST) which contains the matrices of X and Y
variables, i.e. if mydata is such data frame then mydata$expr gives
the expression matrix and mydata$medical is the matrix of medical
data. 
If I do:
>mydata <- data.frame(expr=expr, medical=medical)

I simply obtain a single data.frame combining the two and I am not
able to call the matrices independently  I have been seaching the R
help and documentation without sucess. 
Any help appreciated.

Ana