diff --git a/R/mapping_functions.R b/R/mapping_functions.R
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+#' Map UniProt IDs to NCBI accessions
+#'
+#' This function queries UniProt and returns the corresponding NCBI protein accessions.
+#' If the UniProt API is unreachable, it returns NA values with a warning.
+#'
+#' @param uniprot_ids A character vector of UniProt accession IDs.
+#' @return A tibble with UniProt IDs and mapped NCBI accessions.
+#' @examples
+#' up2ncbi_fast(c("P12345", "Q9Y263"))
+#' @export
+up2ncbi <- function(uniprot_ids) {
+    base_url <- "https://rest.uniprot.org/uniprotkb/search"
+    query <- paste0("accession:(", paste(uniprot_ids, collapse = " OR "), ")")
+
+    resp <- tryCatch(
+        {
+            httr2::request(base_url) |>
+                httr2::req_url_query(query = query, format = "json", fields = "accession,xref_refseq") |>
+                httr2::req_perform()
+        },
+        error = function(e) {
+            warning("UniProt API request failed: ", conditionMessage(e))
+            return(NULL)
+        }
+    )
+
+    if (is.null(resp)) {
+        return(tibble::tibble(uniprot_id = uniprot_ids,
+                              ncbi_accession = NA_character_))
+    }
+
+    results <- httr2::resp_body_json(resp)
+
+    if (length(results$results) == 0) {
+        return(tibble::tibble(uniprot_id = uniprot_ids,
+                              ncbi_accession = NA_character_))
+    }
+
+    df <- tibble::tibble(
+        uniprot_id = vapply(results$results, function(x) x$primaryAccession, ""),
+        ncbi_accession = vapply(results$results, function(x) {
+            if (!is.null(x$uniProtKBCrossReferences)) {
+                paste0(
+                    unique(unlist(lapply(x$uniProtKBCrossReferences, function(y) {
+                        if (y$database == "RefSeq") y$id else NULL
+                    }))),
+                    collapse = ";"
+                )
+            } else NA_character_
+        }, "")
+    )
+
+    df <- dplyr::right_join(df, tibble::tibble(uniprot_id = uniprot_ids), by = "uniprot_id")
+
+    return(df)
+}
+
+#' Map NCBI RefSeq Protein IDs to UniProt Accessions
+#'
+#' This function maps RefSeq protein accessions (from NCBI) to corresponding
+#' UniProt accession IDs using the UniProt ID mapping REST API.
+#'
+#' @param ncbi_ids A character vector of RefSeq protein accessions (e.g. \code{"NP_001026859.1"}).
+#' @param max_retries Integer; maximum number of retries while waiting for the UniProt mapping job (default = 5).
+#' @param wait_time Numeric; number of seconds to wait between retries (default = 3).
+#'
+#' @return A tibble with two columns:
+#' \describe{
+#'   \item{ncbi_id}{Input NCBI RefSeq protein accession(s).}
+#'   \item{uniprot_id}{Mapped UniProt accession(s), or \code{NA} if no mapping was found.}
+#' }
+#'
+#' @details
+#' This function sends a request to the UniProt ID mapping service
+#' (\url{https://rest.uniprot.org/}) and retrieves UniProtKB accessions
+#' corresponding to the given RefSeq protein IDs. If UniProt’s service
+#' is down or results are delayed, the function retries until \code{max_retries}
+#' is reached, after which it returns \code{NA}.
+#'
+#' @examples
+#' \dontrun{
+#' ncbi2up(c("NP_001026859.1", "XP_002711597.1"))
+#' }
+#'
+#' @export
+ncbi2up <- function(ncbi_ids, max_retries = 5, wait_time = 3) {
+    library(httr2)
+    library(dplyr)
+    library(purrr)
+    
+    base_url <- "https://rest.uniprot.org/idmapping/run"
+    status_base <- "https://rest.uniprot.org/idmapping/status/"
+    result_base <- "https://rest.uniprot.org/idmapping/results/"
+    
+    
+    res <- request(base_url) |>
+        req_body_form(
+            from = "RefSeq_Protein",
+            to = "UniProtKB",
+            ids = paste(ncbi_ids, collapse = ",")
+        ) |>
+        req_perform()
+    
+    res_json <- res |> resp_body_json()
+    job_id <- res_json$jobId
+    message("Job submitted. Job ID: ", job_id)
+    
+    
+    attempt <- 1
+    repeat {
+        Sys.sleep(wait_time)
+        status <- request(paste0(status_base, job_id)) |>
+            req_perform() |>
+            resp_body_json()
+        
+        if (isTRUE(status$jobStatus == "FINISHED") || !is.null(status$results)) break
+        
+        if (attempt >= max_retries) {
+            warning("Timeout: Results not ready. Returning NA.")
+            return(tibble(ncbi_id = ncbi_ids, uniprot_id = NA))
+        }
+        
+        message("Waiting for UniProt mapping results... (Attempt ", attempt, ")")
+        attempt <- attempt + 1
+    }
+    
+    # Fetching results
+    result <- request(paste0(result_base, job_id)) |>
+        req_perform() |>
+        resp_body_json()
+    
+    if (is.null(result$results)) {
+        warning("No results returned by UniProt API.")
+        return(tibble(ncbi_id = ncbi_ids, uniprot_id = NA))
+    }
+    
+    #Extract mappings
+    mappings <- map_df(result$results, ~tibble(ncbi_id = .x$from, uniprot_id = .x$to))
+    
+    
+    final <- tibble(ncbi_id = ncbi_ids) |>
+        left_join(mappings, by = "ncbi_id")
+    
+    return(final)
+}
+
+
+#' Map NCBI Protein Accessions to IPG (Identical Protein Group) IDs
+#'
+#' This function maps NCBI protein accession numbers to their corresponding
+#' Identical Protein Group (IPG) IDs using the NCBI Entrez API.
+#' It first attempts to retrieve the link via `rentrez::entrez_link()` and,
+#' if that fails, falls back to a direct E-utilities REST query.
+#'
+#' @param acc A character vector of NCBI protein accession numbers.
+#' @param api_key (optional) NCBI API key to increase rate limits.
+#'
+#' @return A data.frame with columns:
+#' \describe{
+#'   \item{Accession}{Input NCBI protein accession(s)}
+#'   \item{IPG_ID}{Mapped Identical Protein Group ID(s), if available}
+#' }
+#'
+#' @examples
+#' \dontrun{
+#'   acc2ipg(c("WP_000003915.1", "NP_414543.1"))
+#' }
+#'
+#' @export
+acc2ipg <- function(acc, api_key = NULL) {
+    
+    if (!requireNamespace("rentrez", quietly = TRUE)) {
+        stop("Please install 'rentrez' with install.packages('rentrez') or BiocManager::install('rentrez')")
+    }
+    if (!requireNamespace("httr", quietly = TRUE) ||
+        !requireNamespace("xml2", quietly = TRUE)) {
+        stop("Please install 'httr' and 'xml2' packages.")
+    }
+    
+    ipg_from_rest <- function(a) {
+        url <- paste0(
+            "https://eutils.ncbi.nlm.nih.gov/entrez/eutils/esearch.fcgi?",
+            "db=ipg&term=", a, "[Accession]"
+        )
+        res <- httr::GET(url)
+        if (httr::status_code(res) != 200) return(NA)
+        xml <- xml2::read_xml(httr::content(res, "text"))
+        id <- xml2::xml_text(xml2::xml_find_first(xml, ".//IdList/Id"))
+        if (length(id) == 0 || id == "") NA else id
+    }
+    
+    results <- lapply(acc, function(a) {
+        tryCatch({
+            
+            prot_search <- rentrez::entrez_search(
+                db = "protein",
+                term = paste0(a, "[Accession]"),
+                api_key = api_key
+            )
+            
+            if (length(prot_search$ids) == 0) {
+                # fallback to REST
+                ipg_id <- ipg_from_rest(a)
+                return(data.frame(Accession = a, IPG_ID = ipg_id))
+            }
+            
+            
+            link_res <- rentrez::entrez_link(
+                dbfrom = "protein",
+                db = "ipg",
+                id = prot_search$ids[1],
+                api_key = api_key
+            )
+            
+            ipg_id <- if (!is.null(link_res$links$protein_ipg)) {
+                link_res$links$protein_ipg[1]
+            } else {
+                
+                ipg_from_rest(a)
+            }
+            
+            data.frame(Accession = a, IPG_ID = ipg_id)
+        },
+        error = function(e) {
+            message(paste("Error retrieving IPG for", a, ":", e$message))
+            data.frame(Accession = a, IPG_ID = NA)
+        })
+    })
+    
+    do.call(rbind, results)
+}
+
+