What is ARCA?

The term ARCA refers to "Anti-Reverse Cap Analog."

In this modified cap analog, the three prime hydroxyl (3’-OH) group closer to the 7-methyl-guanosine (m7G) is replaced with a methoxy group (–OCH3) group. The anti-reverse cap analog (ARCA: 3´-O-Me-m7G(5 )ppp(5 )G) prevents that synthetic mRNAs will have the cap-0 in the reverse orientation.

Because of the 3'-OMe group, the 
RNA polymerase can only initiate transcription with the remaining hydroxyl group, thus forcing ARCA incorporation in the forward orientation. 

Natural messenger RNA can contain three different caps:

 Cap-0 (m7G(5 )pppN1pN2p),
 Cap-1 (m7G(5 )pppN1mpNp) or
 Cap-2 (m7G(5 )pppN1mpN2mp).

These cap structures differ from each other in the methylation status of the 2'-position of the 5'-penultimate and antepenultimate nucleoside.

Cap analogs enhance mRNA stability and translation efficiency.

All our standard mRNA transcripts have a Cap-1 structure!
Bio-Synthesis also provides ARCA, anti-Reverse Cap Analogs (3' O-Me-M7G(5')ppp(5')G) as an option.
ARCA can only be inserted in the proper orientation which leads to higher translation levels.