There are two types of mustard plant with defined allergens, Oriental Mustard and Yellow Mustard.
Oriental mustard only contains one allergen, Bra j 1, which is a 2S seed storage protein commonly found in nuts and seeds.
Yellow mustard has 4 food allergens. Sin a 1 is also a 2S seed storage protein and Sin a 2 is an 11S seed storage protein. Sin a 3 is a lipid transfer protein (LTP) and Sin a 4 is a profilin protein.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Mustard.php
The allergy resources page for mustard covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with mustard.
There is also regularly updated links to useful websites specifically for mustard allergies and the latest research papers on the topic.
If you have multiple allergies and are interested in how they are linked or want to find out more about food allergies outside the usual top 14 then you can follow me on Facebook or Twitter, where I share a food allergy card every day of the week.