hi everybody, thanks a lot for all the useful information you have kindly provided. here is how i have fixed the problem using most of the info you have provided: FIRST STEP: > temp <- eapply(hgu133aMAP, paste, sep = "", collapse = ";") > temp <- cbind(names(temp), unlist(temp)) > temp <- AnnBuilder:::getReverseMapping(temp) > map2Probe <- cbind(names(temp), unlist(temp)) # (as kindly suggested by John Zhang). # this returns map2Probe which is a matrix of dimension 2512x2 with cytobande name in column 1 and probesetIDs in column2. SECOND STEP: > library(graph) > h<-hashtable() #i have created a hash table using hashtable() from the library(graph). THIRD STEP: > for (i in 1:2512) > h[[map2Probe[i,1]]]<-unlist(strsplit(map2Probe[i,2], "\\;")) #this populate the hash table with each element of the matrix map2Probe using the element in column 1 as "key" and the element in column "2" as "value" ("key" and "value" are from Perl terminology) and splits the "value" string substituting the ";" between each probesetID. i am sure there are nicer and quicker ways, but anyway, i hope this can be useful to someone in the future. thanks again, cheers Dario -- Dario Greco Institute of Biotechnology - University of Helsinki Building Cultivator II P.O.Box 56 Viikinkaari 4 FIN-00014 Finland Office: +358 9 191 58951 Fax: +358 9 191 58952 Mobile: +358 44 023 5780 Lab WebPage: http://www.biocenter.helsinki.fi/bi/dna-microarray/ Personal WebPage: http://www.biocenter.helsinki.fi/bi/dna-microarray/dario.htm