diff --git a/inst/book/tenx-filtered-pbmc3k-4k-8k.Rmd b/inst/book/tenx-filtered-pbmc3k-4k-8k.Rmd
index 6b4e4f7..92f5eb4 100644
--- a/inst/book/tenx-filtered-pbmc3k-4k-8k.Rmd
+++ b/inst/book/tenx-filtered-pbmc3k-4k-8k.Rmd
@@ -37,7 +37,7 @@ library(scater)
 stats <- high.mito <- list()
 for (n in names(all.sce)) {
     current <- all.sce[[n]]
-    is.mito <- grep("MT", rowData(current)$Symbol_TENx)
+    is.mito <- grep("^MT-", rowData(current)$Symbol_TENx)
     stats[[n]] <- perCellQCMetrics(current, subsets=list(Mito=is.mito))
     high.mito[[n]] <- isOutlier(stats[[n]]$subsets_Mito_percent, type="higher")
     all.sce[[n]] <- current[,!high.mito[[n]]]