simple-bases 

A string of simple nucleotide bases.  These are A, C, G and T (DNA).  
Used to define reference bases and alternate bases for sequence 
variants.

Simple Type Information

Used By

Attributes variant/@alternate-bases, variant/@reference-bases

Source 

<xs:simpleType name="simple-bases">
  <xs:annotation>
    <xs:documentation>A string of simple nucleotide bases. These are A, C, G and T (DNA). Used to define reference bases and alternate bases for sequence variants.</xs:documentation>
  </xs:annotation>
  <xs:restriction base="xs:string">
    <xs:pattern value="([\sACGTUacgtu])+"/>
    <xs:minLength value="1"/>
  </xs:restriction>
</xs:simpleType>