R : problem with write.table and a dataframe

Question

I have a dataframe (mydataframe1) which looks like this :

      gene      expression         description
...     ...         ...              ...
112   G1246       2.4567      Heat shock protein
113   G1389      -1.7491      Heat regulation
114   G1467       0.1234      Cell component
...     ...          ...              ...
753   G1923       0.9435      Heat adaptative synthesis
...     ...          ...              ...
2843  G2123       1.4677      Immune response

I'm interested in genes having "heat" in their description, to get the concerned lines, I used the following script :

mydataframe2 <- mydataframe1[grep("heat_?", mydataframe1$description, ignore.case = TRUE),]

I get a dataframe (mydataframe2) with the corresponding rows, but the row numbering is the same as in the original dataframe

      gene      expression         description
112   G1246       2.4567      Heat shock protein
113   G1389      -1.7491      Heat regulation
753   G1923       0.9435      Heat adaptative synthesis

I save this like this :

write.table(mydataframe2, "/my/path/heat_gene.csv", row.names=FALSE , col.names=TRUE, sep="\t", dec=".", quote=FALSE) 

But when I open heat_gene.csv, I see that there are still all the rows of mydataframe1.

Do you know why?