Cai et al. reply

Replying to: V. M. Christoffels et al. , 10.1038/nature07916 (2009) We recognized the importance of confirming data from Cre lineage studies with an alternative approach, and thus used a fluorescent dye to lineage-label epicardial cells. This approach confirmed that epicardial cells contributed to myocytes within the heart1. In our experiments, we never observed Tbx18 mRNA within ventricles at embryonic day E10.5 to E11.5, although our RNA signal in epicardium at these stages is at least as strong as that observed by Christoffels et al.2. We note a discrepancy between amount of RNA staining (broadly throughout the middle of the ventricle) and protein staining (much more restricted) that Christoffels et al.2 observe at early stages. Reasons for discrepancies between our RNA in situ data and that of Christoffels et al. are unclear. Christoffels et al.2 detect Tbx18 intronic sequences by RT–PCR of E11.5 IVS, but do not provide data to demonstrate mature Tbx18 mRNA. Notably, we did observe active expression of Tbx18 in non-myocytes after E11.5 (ref. 1).