Table 1 Overview of the design and most important results of studies focusing on the prognostic and/or predictive value of basal cytokeratin expression in invasive breast cancer

Authors	Source of tissue	Clinical data	Type of basal cytokeratin	Threshold for the expression of basal cytokeratins	Conclusion
Rakha et al51	TMA	Not specified in detail	Ck 5/6 and/or Ck 14	10%	Expression of Ck 5/6 and/or Ck 14 is sufficient for the definition of basal breast cancer

Jumpannen et al40	TMA	No adjuvant treatment	Ck 5/Ck 14/p63 antibody cocktail	20%	No impact on overall survival in ER-negative breast cancers

Abd El-Rehim et al28	TMA	Not specified in detail	Ck 5/6 and/or Ck 14	Any positive cell	Expression of basal cytokeratins is an independent prognostic marker At least four different immunohistochemical phenotypes can be defined

Laakso et al39	Tissue sections	(a) without adjuvant treatment (b) dose-escalated adjuvant chemotherapy	Ck 5/Ck 14/p63 antibody cocktail	5% 70% for the “true” basal subtype	The true basal breast cancer subtype in contrast to the basoluminal subtype is not associated with HER2-amplifications and showed a significantly different prognosis No prognostic value of the expression of basal cytokeratins in high-risk patients

Van de Rijn et al79	TMA	Patients with and without adjuvant chemotherapy, and anti-hormone therapy, respectively	Ck 17 and/or Ck 5/6	Invasive tumour cells with strong staining	Basal cytokeratin expression was an independent prognostic marker in node-negative breast cancer

Nielsen et al45	TMA	Patients with adjuvant and neoadjuvant chemotherapy, anti-hormone therapy, respectively	Ck 17 and/or Ck 5/6	Invasive tumour cells with <20% of weak staining	A panel of 4 antibodies (ER, Ck 5/6, HER2 and EGFR) can accurately identify basal-like tumours

Jones et al38	Tissue sections of grade 3 breast cancers	Not specified in detail	Ck 14	Any positive cell	Basal carcinomas have a decreased overall survival and show a distinct chromosomal alteration pattern

Fulford et al42	Tissue sections of grade 3 breast cancers	Not specified in detail	Ck 14	At least 1% of invasive tumour cells	Basal carcinomas develop less bone and liver metastasis. Overall survival was improved in basal grade 3 carcinomas compared to non-basal grade 3 carcinomas. In metastatic disease the prognosis was decreased

Banerjee et al80	Tissue sections of grade 3 breast cancers	Not specified in detail	Ck 5/6, Ck 14, Ck 17	Any positive cell	Significantly increased recurrence rate and overall survival Adjuvant anthracyline-based chemotherapy seems less effective compared to grade matched controls

TMA, tissue microarray.