PTK6 (BRK) phosphorylates CDKN1B (p27KIP1) bound to the complex of CDK4 and CCND1 (cyclin D1) on tyrosine residue Y88 and possibly other tyrosines (e.g. Y89) (Patel et al. 2015). Based on the finding that PTK6 promotes ERBB2-induced increase in cyclin E1 (CCNE1) levels and decrease in CDKN1B levels (Xiang et al. 2008), and supported by the analogy with other SRC family kinases that phosphorylate CDKN1B (Grimmler et al. 2007), PTK6 is likely to also phosphorylate CDKN1B bound to the complex of CCNE1 and CDK2. Phosphorylation of CDKN1B (p27KIP1) on tyrosine residue Y88 by SRC family kinases dislodges the 3-10 helix of CDKN1B from the active site of CDK2 or CDK4, thus converting CDKN1B from a bound inhibitor to a bound non-inhibitor (Grimmler et al. 2007, Ray et al. 2009).