Development of an efficient cancer control program is essential, considering that the incidence of cancer is increasing in Korea. Cancer registration and cancer screening are important cancer control programs that are closely related to and influenced by each other. The hospital-based cancer registry started in 1980 with 47 training hospitals participating in the cancer control program in Korea. Currently, the registry includes 80-90% of cancer cases from more than 150 training hospitals. The details of the history, objectives, and activities of the Korea Central Cancer Registry (KCCR) have been documented in 2005 and 2011.1,2 Cancer cases are classified according to the International Classification of Diseases for Oncology, 3rd edition (ICD-O-3)3 and then converted according to the International Classification of Diseases, 10th edition (ICD-10).4 As discussed in our first proposal,5 the roles of pathologists are important for improving the quality of cancer statistics since they provide a correct diagnosis and classification of the cancer which is essential for the application of the ICD-O code, in particular the behavior code. In collaboration with the National Cancer Center, the Korean Society of Pathologists (KSP) has participated in confirmation of diagnostic terms, standardization of diagnostic formats, clarification and assessment of multiple primaries, primary sites and the ICD-O code, and education of the pathologists. In addition, the KSP has also contributed to the education of cancer registrars because they play a key role in entering the data in the cancer registry. We have previously noticed the differences in the diagnostic terms between pathologists and the ICD-O code book. It is likely that these differences may originate from numerous coexisting classification systems, synonyms, new entities, newly recognized tumor behavior, and time interval between identification of an entity and its application to the code book. Of these, clarification of the behavior code is important for the registry, because behavior code 2 (carcinoma in situ) and 3 (invasive carcinoma and sarcoma) must be registered and used for both cancer statistics and insurance reimbursement. It is noteworthy, however, that some tumors including microinvasive tumors of the breast and ovary are not included in the ICD-O code book.3 The Gastrointestinal Pathology Study Group of the KSP therefore proposed behavior codes for several gastrointestinal tumors in 2008.5 Whether a microinvasive tumor (especially diagnosed as ductal carcinoma in situ with microinvasion [DCISM]) of the breast should be treated as carcinoma in situ or invasive carcinoma has been an important issue. The behavior of microinvasive tumors remains undetermined. Therefore, there is a controversy regarding this matter even among specialists.6-14 In addition, some clinicians and pathologists don't have exact concept about this matter. Furthermore, there is even a controversy regarding how to abbreviate microinvasive tumor into DCISM or microinvasive carcinoma (MIC) between the pathologists. This poses a problem to the registrars when they should enter the data in the cancer registry. An appropriate behavior code can be assigned only when they understand the meaning of different pathologic terminology. It would therefore be necessary not only to standardize the pathologic terminologies but also to have an identical understanding of the biologic behavior of the tumor, which is essential for the registration of tumors. In addition, borderline serous or mucinous tumors are issues that remain unresolved in association with diagnostic criteria, diagnostic terminology, behavior and treatment.15-21 We have therefore made an additional proposal of behavior codes for microinvasive tumors of the breast and ovary based on our previous proposal. In addition, we have also focused on the clinically meaningful behavior code rather than diagnostic criteria. Given the above background, we made our second proposal. But this is not conclusive but subject to alterations with the accumulation of more experience and knowledge. However, reconsideration and understanding of the biological behavior of microinvasive tumors of the breast and ovary and sharing a common concept will be helpful in statistics and in changing after amending the rule. Thus, we would like to report a current progress on our second proposal.