The only serological marker present during window period in hepatitis B infection is?

Anti HBc REF: Harrison's Internal Medicine 171}t edition chapter 298 (see Appendix-28 for "Markers of hepatitis-B) Variability exists in the time of appearance of anti-HBs after HBV infection, occasionally a gap of several weeks or longer may separate the disappearance of HBsAg and the appearance of anti-HBs. During this "gap" or "window" period, anti-HBc may represent the only serologic evidence of current or recent HBV infection, and blood containing anti-HBc in the absence of HBsAg and anti-HBs has been implicated in the development of transfusion-associated hepatitis B