The sequence number field is used for the reliable delivery of all messages.
It is not mandatory in GTP' and the reason is it has one octet sub field in header named as N-PDU number. which serves the same purpose if SN flag is not set.
In case of GTPv2 sequence number field is mandatory and there is nothing like N-PDU number. It contains from Octet 9 to Octet 11 of header part.
So in case of Iuh interface, it should be present in all messages if it's using GTPv2.