Area for retaining behavior portion (1 digit) of the ICD-O-1 or field trial morphology codes entered before a conversion to ICD-O-2. The item name includes years 73-91. However, some states may have used the codes for cases before 1973. It is a subfield of the morphology code.

For tumors diagnosed before 1992, contains the ICD-O-1 or field trial 1-digit behavior code as originally coded, if available. Blank for tumors coded directly into a later version of ICD-O.