“…The ecto‐protein kinase is primarily localized in the acrosomal cap area of the external surface of the mature sperm head as demonstrated by indirect immunofluorescence studies (Nath et al, 2008). “Ecto” nature of CIK as well as MPS has been established conclusively using indirect immunofluorescence studies based on antibodies of these pure antigens and studies on activities of these molecules utilizing their monovalent antibodies (Maiti et al, 2004, 2008; Nath et al, 2008). Although earlier investigators have provided several lines of evidences for the occurrence of ecto‐protein kinases in a variety of mammalian cells (Mastro and Rosengurt, 1976; Schlaeger and Kohler, 1976; Majumder, 1981; Halder and Majumder, 1986; Dey and Majumder, 1990; Walter et al, 2000; Guthmann et al, 2002; for review Nath et al, 2008), the above‐mentioned findings from our laboratory provide confirmatory evidence for the localization of an ecto‐protein kinase and its protein substrate on a cell surface.…”