How do I remove leading comma in SQL query results?

Hello. Below is my query from SQL query analyzer.

Question: How to rewrite to remove leading "," from query results? The current results are " , 234, 345" etc.

Thanks.

Declare @PacklistID varchar (15)
Declare @LineNum numeric
Declare @SerNumList varchar(1000)

SET @SerNumList = ''

select @PacklistID=PACKLIST_ID, @LineNum=LINE_NUM,@SerNumList = @SerNumList + ', ' + SERIAL_NUM
from APG_SN_DETAIL

insert into APG_SN_SUMMARY(PACKLIST_ID, LINE_NUM, SERIAL_LIST)VALUES(@PacklistID,@LineNum,@SerNumList)